This is the 29th day of my participation in Gwen Challenge
140000 | 400 multichannel JavaScript 🎓 interview questions with answers 🌠 items (the first part 1-100)
140000 | 400 multichannel JavaScript 🎓 interview questions with answers 🌠 items part ii (101-200)
140000 | 400 multichannel JavaScript 🎓 interview questions with answers 🌠 items (third part 201-300)
140000 | 400 multichannel JavaScript 🎓 interview questions with answers 🌠 items (the fourth part 301-370)
140000 | 400 multichannel JavaScript 🎓 interview questions with answers 🌠 items (the fifth part 371-424)
In order to make it easy to read, I have put links back to the top or to the corresponding questions where necessary
| Serial number | The problem |
|---|---|
| 1 | What are the possible ways to create objects in JavaScript? |
| 2 | What is a prototype chain? |
| 3 | What’s the difference between call, apply and bind? |
| 4 | What is JSON and its common operations? |
| 5 | What is the purpose of the array slice() method? |
| 6 | What is the purpose of the array splice() method? |
| 7 | What’s the difference between slice() and splice()? |
| 8 | How do you compare an Object and a Map |
| 9 | What is the difference between the == and === operators? |
| 10 | What is a lambda or arrow function? |
| 11 | What is a first-class function? |
| 12 | What is a first order function? |
| 13 | What is a higher-order function? |
| 14 | What is a function of one variable? |
| 15 | What is a Currie function? |
| 16 | What is a pure function? |
| 17 | What is the let keyword used for? |
| 18 | What is the difference between let and var? |
| 19 | Why choose the name let as the keyword? |
| 20 | How do you redeclare variables in switch blocks without errors? |
| 21 | What is a temporary dead zone? |
| 22 | What is IIFE (immediately executed function expression)? |
| 23 | What are the benefits of using modules? |
| 24 | What is memoization? |
| 25 | What is the power of variable promotion? |
| 26 | What are classes in ES6? |
| 27 | What is closures? |
| 28 | What is a module? |
| 29 | Why do you need modules? |
| 30 | What is the scope in javascript? |
| 31 | What is a Service Worker? |
| 32 | How do I manipulate the DOM using a Service Worker? |
| 33 | How do you reuse information when a Service Worker restarts? |
| 34 | What is IndexedDB? |
| 35 | What is Web Storage? |
| 36 | What is a Post message? |
| 37 | What is a Cookie? |
| 38 | Why do I need cookies? |
| 39 | What are the options in the cookie? |
| 40 | How do you delete cookies? |
| 41 | What is the difference between cookies, local storage, and session storage? |
| 42 | What are the main differences between localStorage and sessionStorage? |
| 43 | How do you access web storage? |
| 44 | What are the methods available for session storage? |
| 45 | What are stored events and their event handlers? |
| 46 | Why do YOU need Web storage? |
| 47 | How do you check Web storage browser support? |
| 48 | How do you check Web Workers browser support? |
| 49 | Take the example of a Web worker |
| 50 | What are the limitations of Web workers on DOM? |
| 51 | What is promise? |
| 52 | Why do we need Promise? |
| 53 | What are the three states of promise? |
| 54 | What is a callback function? |
| 55 | Why do we need a correction? |
| 56 | What is callback hell? |
| 57 | What is a server sending event? |
| 58 | How do you receive event notifications from the server? |
| 59 | How do you check browser support for sending events to the server? |
| 60 | What are the available events for events sent by the server? |
| 61 | What are the main rules of promise? |
| 62 | What is the callback of a callback? |
| 63 | What is promise chaining? |
| 64 | What is promise.all? |
| 65 | What is the purpose of the race method in a promise? |
| 66 | What are strict patterns in javascript? |
| 67 | Why do you need strict patterns? |
| 68 | How do you declare strict patterns? |
| 69 | What is the purpose of the double exclamation mark? |
| 70 | What is the purpose of deleting an operator? |
| 71 | What is a Typeof operator? |
| 72 | What is an undefined attribute? |
| 73 | What is a null value? |
| 74 | What’s the difference between null and undefined? |
| 75 | What is Eval? |
| 76 | What’s the difference between Window and Document? |
| 77 | How do you access history in javascript? |
| 78 | How do you detect if the caps lock key is on? |
| 79 | What is isNaN? |
| 80 | What is the difference between an undeclared variable and an undefined variable? |
| 81 | What is a global variable? |
| 82 | What’s wrong with global variables? |
| 83 | What is a NaN attribute? |
| 84 | What is the purpose of isFinite? |
| 85 | What is an event flow? |
| 86 | What is event bubbling? |
| 87 | What is Event capturing? |
| 88 | How do you submit forms using JavaScript? |
| 89 | How do you find operating system details? |
| 90 | What’s the difference between a Document Load and a DOMContentLoaded event? |
| 91 | What is the difference between Native, Host and User objects? |
| 92 | What tools or techniques are available for debugging JavaScript code? |
| 93 | What are the advantages and disadvantages of Promises versus corrections? |
| 94 | What’s the difference between attribute and property? |
| 95 | What is the same-Origin policy? |
| 96 | What is the purpose of void 0? |
| 97 | Is JavaScript a compiled or interpreted language? |
| 98 | Is JavaScript a case sensitive language? |
| 99 | Is there a relationship between Java and JavaScript? |
| 100 | What is an event? |
| 101 | Who created JavaScript? |
| 102 | B: What’s the preventDefault method for? |
| 103 | What is the use of the stopPropagation method? |
| 104 | What steps does return False involve? |
| 105 | What is BOM(Browser Object Model)? |
| 106 | What does setTimeout do? |
| 107 | What does setInterval do? |
| 108 | Why is JavaScript considered single-threaded? |
| 109 | What is Event Delegation? |
| 110 | What is ECMAScript? |
| 111 | What is JSON? |
| 112 | What are the syntax rules for JSON? |
| 113 | What is the purpose of JSON stringing? |
| 114 | How do you parse JSON strings? |
| 115 | Why do YOU need JSON? |
| 116 | What is PWA? |
| 117 | What is the purpose of the clearTimeout method? |
| 118 | What is the purpose of the clearInterval method? |
| 119 | How do you redirect new pages in javascript? |
| 120 | How do you check if a string contains a substring? |
| 121 | How do you validate email in javascript? |
| 122 | How do you get the current URL using javascript? |
| 123 | What are the various URL properties of the Location object? |
| 124 | How do I get query string values in javascript? |
| 125 | How do I check if there are keys in an object? |
| 126 | How do you loop or enumerate javascript objects? |
| 127 | How do you test an empty object? |
| 128 | What is a parameter object? |
| 129 | How do you capitalize the first letter of a string? |
| 130 | What are the pros and cons of the for loop? |
| 131 | How do you display the current date in javascript? |
| 132 | How do you compare two date objects? |
| 133 | How do you check if a string starts with another string? |
| 134 | How do you trim strings in javascript? |
| 135 | How do you add a key-value pair to javascript? |
| 136 | ‘! Does the ‘symbol represent a special operator? |
| 137 | How do you assign default values to variables? |
| 138 | How do you define multi-line strings? |
| 139 | What is app Shell Model? |
| 140 | Can we define properties for functions? |
| 141 | What is the way to find the desired number of arguments for a function? |
| 142 | What is polyfill? |
| 143 | What are break and continue statements? |
| 144 | What are JS tags? |
| 145 | What are the benefits of putting statements first? |
| 146 | What are the benefits of initializing variables? |
| 147 | What are the suggestions for creating new objects? |
| 148 | How do you define JSON arrays? |
| 149 | How do you generate random integers? |
| 150 | Can you write a random integer function to print integers in range? |
| 151 | What is tree shaking? |
| 152 | What does it take to tree shaking? |
| 153 | Is eval recommended? |
| 154 | What is a regular expression? |
| 155 | What are the string methods available in regular expressions? |
| 156 | What are modifiers in regular expressions? |
| 157 | What is a regular expression pattern? |
| 158 | What is a RegExp object? |
| 159 | How do I search for patterns in strings? |
| 160 | What is the purpose of the EXEC method? |
| 161 | How do I change the style of an HTML element? |
| 162 | What is the result of 1+2+’3′? |
| 163 | What are debugger statements? |
| 164 | What is the purpose of debugging breakpoints? |
| 165 | Can I use reserved words as identifiers? |
| 166 | How do you detect mobile browsers? |
| 167 | How do I detect mobile browsers without regular expressions? |
| 168 | How do you get the width and height of an image using JS? |
| 169 | How do I synchronize HTTP requests? |
| 170 | How do I make an asynchronous HTTP request? |
| 171 | How do you convert dates to another time zone in javascript? |
| 172 | What are the attributes used to get the window size? |
| 173 | What are conditional operators in javascript? |
| 174 | Can you apply chaining to conditional operators? |
| 175 | What are the ways to execute javascript after a page loads? |
| 176 | What’s the difference between Proto and Prototype? |
| 177 | For example, you really need a semicolon |
| 178 | What is the Freeze method? |
| 179 | What is the purpose of the Freeze method? |
| 180 | Why do I need to use freeze? |
| 181 | How do you detect browser language preferences? |
| 182 | How do I use javascript to convert a string to title case? |
| 183 | How do you detect disabled javascript on a page? |
| 184 | What are the various operators supported by javascript? |
| 185 | What are REST parameters? |
| 186 | What happens if you don’t use the REST parameter as the last parameter? |
| 187 | What are bitwise operators available in JavaScript? |
| 188 | What is the spread operator? |
| 189 | How do you determine if an object is frozen? |
| 190 | How do you determine if two values are the same or do not use objects? |
| 191 | What is the purpose of the IS method using Object? |
| 192 | How do I copy properties from one object to another? |
| 193 | What are the applications of assignment methods? |
| 194 | What is a Proxy object? |
| 195 | What is the purpose of the SEAL method? |
| 196 | What are the applications of the SEAL method? |
| 197 | What is the difference between the Freeze and SEAL methods? |
| 198 | How do I determine whether an object is SEAL? |
| 199 | How do I get enumerable key-value pairs? |
| 200 | What are the main differences between the object. values and object. entries methods? |
| 201 | How do I get the list of keys for any object? |
| 202 | How do you create an object with Prototype? |
| 203 | What is WeakSet? |
| 204 | What is the difference between WeakSet and Set? |
| 205 | List the set of methods available on WeakSet? |
| 206 | What is WeakMap? |
| 207 | What is the difference between a WeakMap and a Map? |
| 208 | List the set of methods available on WeakMap? |
| 209 | What is the purpose of Uneval? |
| 210 | How do you encode a URL? |
| 211 | How do you decode a URL? |
| 212 | How do you print the content of a web page? |
| 213 | What’s the difference between uneval and eval? |
| 214 | What is an anonymous function? |
| 215 | What is the priority of local and global variables? |
| 216 | What is a javascript accessor? |
| 217 | How do you define properties on object constructors? |
| 218 | What’s the difference between GET and defineProperty? |
| 219 | What are the advantages of getters and setters? |
| 220 | Can I add getters and setters using the defineProperty method? |
| 221 | What is the purpose of switch-case? |
| 222 | What conventions do YOU follow when using Swtich Case? |
| 223 | What is a primitive data type? |
| 224 | What are the different ways to access object properties? |
| 225 | What are function argument rules? |
| 226 | What is an error object? |
| 227 | When you receive a syntax error |
| 228 | What are the different error names for error objects? |
| 229 | What are the various statements in error handling? |
| 230 | What are the two types of loops in javascript? |
| 231 | What is NodeJS? |
| 232 | What is an Intl object? |
| 233 | How do you perform language-specific date and time formatting? |
| 234 | What is an iterator? |
| 235 | How does synchronous iteration work? |
| 236 | What is an event loop? |
| 237 | What is the call stack? |
| 238 | What is an event queue? |
| 239 | What is a decorator? |
| 240 | What are the properties of the Intl object? |
| 241 | What is a unary operator? |
| 242 | How do I sort the elements of an array? |
| 243 | What is the purpose of compareFunction when sorting arrays? |
| 244 | How do you invert an array? |
| 245 | How do you find minimum and maximum values in an array? |
| 246 | How do I find min and Max values without math functions? |
| 247 | What is a null statement and its purpose? |
| 248 | How do I get metadata for a module? |
| 249 | What is a comma operator? |
| 250 | What are the advantages of the comma operator? |
| 251 | What is typescript? |
| 252 | What’s the difference between javascript and typescript? |
| 253 | What’s typescript’s advantage over javascript? |
| 254 | What is an object initializer? |
| 255 | What is a constructor? |
| 256 | What happens if you write a constructor more than once in a class? |
| 257 | How do I call the constructor of a parent class? |
| 258 | How do you get a prototype of an object? |
| 259 | What happens if I pass a string type for the getPrototype method? |
| 260 | How do I set the stereotype of one object to another? |
| 261 | How do you check if an object is extensible? |
| 262 | How do I prevent objects from expanding? |
| 263 | What are the different ways to make an object unextensible? |
| 264 | How can I define multiple attributes on an object? |
| 265 | What is MEAN in JavaScript? |
| 266 | What is obfuscation in javascript? |
| 267 | Why confusion? |
| 268 | What is Minification? |
| 269 | What are the benefits of shrinking? |
| 270 | What’s the difference between obfuscation and encryption? |
| 271 | What are the commonly used miniaturization tools? |
| 272 | How do you perform form validation using javascript? |
| 273 | How do I perform form validation without javascript? |
| 274 | What are the DOM methods available for constraint validation? |
| 275 | What are the constraints available to validate DOM attributes? |
| 276 | What is a validity attribute list? |
| 277 | For example using the rangeOverflow property? |
| 278 | Is enumeration provided in javascript? |
| 279 | What is enumeration? |
| 280 | How do you list all the attributes of an object? |
| 281 | How do I get the property descriptor of an object? |
| 282 | What properties do property descriptors provide? |
| 283 | How do you extend classes? |
| 284 | How can I change the URL without reloading the page? |
| 285 | How do I check if an array contains a specific value? |
| 286 | How do you compare scalar arrays? |
| 287 | How do I get a value from a get parameter? |
| 288 | How do you print numbers using commas as thousands separators? |
| 289 | What’s the difference between Java and javascript? |
| 290 | Does javascript support namespaces? |
| 291 | How do you declare namespaces? |
| 292 | How do I call javascript code in iframe from the parent page? |
| 293 | How do I get a time zone offset from a date? |
| 294 | How to load CSS and JS files dynamically? |
| 295 | What are the different ways to find HTML elements in the DOM? |
| 296 | What is jQuery? |
| 297 | What is the V8 JavaScript engine? |
| 298 | Why do we call javascript a dynamic language? |
| 299 | What is an air freight operator? |
| 300 | How do I set cursor wait? |
| 301 | How do you create an infinite loop? |
| 302 | Why should we avoid the with statement? |
| 303 | What is the output of the following for loop? |
| 304 | List some features of ES6? |
| 305 | What is ES6? |
| 306 | Can I redeclare let and const variables? |
| 307 | Does a const variable make the value immutable? |
| 308 | What are the default parameters? |
| 309 | What are template literals? |
| 310 | How do I write multiline strings in template text? |
| 311 | What is a nested template? |
| 312 | What is a tag template? |
| 313 | What is a primitive string? |
| 314 | What is deconstruction assignment? |
| 315 | What is the default value in destructuring assignment? |
| 316 | How do you swap variables in a deconstruction assignment? |
| 317 | What are enhanced object literals? |
| 318 | What is dynamic import? |
| 319 | What are dynamically imported use cases? |
| 320 | What is typed arrays? |
| 321 | What are the advantages of a module loader? |
| 322 | What is collation? |
| 323 | What is a… Of the statement? |
| 324 | What is the output of the following expanded array of operators? |
| 325 | Is PostMessage secure? |
| 326 | What’s wrong with postMessage target sources being wildcards? |
| 327 | How do you avoid receiving postMessages from attackers? |
| 328 | Can I avoid Using postMessages entirely? |
| 329 | Is postMessages synchronized? |
| 330 | What is the paradigm of Javascript? |
| 331 | What is the difference between internal and external javascript? |
| 332 | Is JavaScript faster than server-side scripting? |
| 333 | How do you get the status of check boxes? |
| 334 | What is the purpose of the double tilde operator? |
| 335 | How do you convert characters to ASCII? |
| 336 | What is an array buffer? |
| 337 | What is the output of the following string expression? |
| 338 | What is the purpose of an Error object? |
| 339 | What is the purpose of an EvalError object? |
| 340 | What is the list of case errors thrown from non-strict to strict modes? |
| 341 | Are all objects prototypes? |
| 342 | What’s the difference between arguments and arguments? |
| 343 | What is the purpose of the some method in an array? |
| 344 | How do you combine two or more arrays? |
| 345 | What is the difference between shallow copy and deep copy? |
| 346 | How do you create a specific number of copies of a string? |
| 347 | How do you return all matched strings based on a regular expression? |
| 348 | How do you trim strings at the beginning or end? |
| 349 | What is the output of the following console statement with unary operator? |
| 350 | Does javascript use mixins? |
| 351 | What is the Thunk function? |
| 352 | What is asynchronous thunk? |
| 353 | What is the output of the following function call? |
| 354 | How do I remove all newlines from a string? |
| 355 | What’s the difference between reflow and redraw? |
| 356 | What happens when you negate an array? |
| 357 | What happens if we add two arrays? |
| 358 | What is the output of adding an addition operator to a false value? |
| 359 | How to use special characters to create from strings? |
| 360 | How do I remove spurious values from an array? |
| 361 | How do you get the unique value of an array? |
| 362 | What is a deconstruction alias? |
| 363 | How do I map array values without using the Map method? |
| 364 | How do you empty an array? |
| 365 | How do you round numbers to certain decimals? |
| 366 | What’s the easiest way to turn an array into an object? |
| 367 | How do you create an array with some data? |
| 368 | What is the placeholder for the console object? |
| 369 | Can I add CSS to console messages? |
| 370 | What is the purpose of the dir method on a console object? |
| 371 | Can I debug HTML elements in the console? |
| 372 | How do I display data in tabular format using console objects? |
| 373 | How do you verify that the parameter is a number? |
| 374 | How do you create a copy to Clipboard button? |
| 375 | What is the shortcut to get the timestamp? |
| 376 | How do you flatten a multidimensional array? |
| 377 | What is the simplest multi-condition check? |
| 378 | How do you capture the browser back button? |
| 379 | How do I disable the right click in a web page? |
| 380 | What is a wrapper object? |
| 381 | What is AJAX? |
| 382 | What are the different ways to handle asynchronous code? |
| 383 | How do I cancel an extract request? |
| 384 | What is voip API? |
| 385 | What is the minimum timeout limit? |
| 386 | How do you achieve zero timeouts in modern browsers? |
| 387 | What are the tasks in the event loop? |
| 388 | What are microtasks? |
| 389 | What are the different event cycles? |
| 390 | What is the purpose of queueMicrotask? |
| 391 | How do you use javascript libraries in typescript files? |
| 392 | Promises and observables what is the difference between Promises and observables? |
| 393 | What is a heap? |
| 394 | What is an event table? |
| 395 | What is a microtask queue? |
| 396 | What is the difference between Shim and Polyfill? |
| 397 | How do you detect primitive or non-primitive value types? |
| 398 | What is Babel? |
| 399 | Is Node.js completely single-threaded? |
| 400 | What are common use cases for observables? |
| 401 | What is RxJS? |
| 402 | What is the difference between a function constructor and a function declaration? |
| 403 | What is a short circuit? |
| 404 | What’s the easiest way to resize an array? |
| 405 | What is observable? |
| 406 | What is the difference between a function declaration and a class declaration? |
| 407 | What is an asynchronous function? |
| 408 | How would you prevent Promises swallowing errors? |
| 409 | What is Deno? |
| 410 | How do you make objects iterable in javascript? |
| 411 | What is the correct tail call? |
| 412 | How do you check if an object is a promise? |
| 413 | How do I detect if a function is called as a constructor? |
| 414 | What is the difference between arguments objects and REST parameters? |
| 415 | What is the difference between an extension operator and a REST parameter? |
| 416 | What are the different types of generators? |
| 417 | What is a built-in iterable? |
| 418 | for… Of and the for… What are the differences between in statements? |
| 419 | How do I define instance and non-instance attributes? |
| 420 | What is the difference between isNaN and number. isNaN? |
| 421 | How do I call IIFE without any extra parentheses? |
| 422 | Can expressions be used in switch cases? |
| 423 | What’s the easiest way to ignore a promise error? |
| 424 | How do I style console output using CSS? |
201. How do I get the key list of any object?
You can use the methods object.keys () uses to return an array of the given Object’s own property names, in the same order as we would get with a normal loop. For example, you can get the key of a user object,
const user = {
name: 'Haiyong'.gender: 'male'.age: 40
};
console.log(Object.keys(user)); //['name', 'gender', 'age']
Copy the codeLet’s go back to the top and we’re on problem 100 and we’re on problem 200
202. How do you create an object with Prototype?
The object.create () method is used to create a new Object with the specified prototype Object and properties. That is, it uses an existing object as a prototype for a newly created object. It returns a new object with the specified prototype object and properties.
const user = {
name: 'Haiyong'.printInfo: function () {
console.log(`My name is The ${this.name}. `); }};const admin = Object.create(user);
admin.name = "Nick"; // Remember that "name" is a property set on the "admin" object, not the "user" object
admin.printInfo(); // My name is Nick
Copy the codeLet’s go back to the top and we’re on problem 100 and we’re on problem 200
203. What is WeakSet?
WeakSet is used to store a collection of weakly (weak reference) held objects. Syntax is as follows,
new WeakSet([iterable]);
Copy the codeLet’s look at the following example to explain its behavior,
var ws = new WeakSet(a);var user = {};
ws.add(user);
ws.has(user); // true
ws.delete(user); // Delete the user from the collection
ws.has(user); // false, user has been deleted
Copy the codeLet’s go back to the top and we’re on problem 100 and we’re on problem 200
204. What is the difference between a WeakSet and a Set?
The main difference lies in that the object reference in Set is a strong reference, while the object reference in WeakSet is a weak reference. That is, objects in WeakSet can be garbage collected if there are no other references.
The other difference is,
WeakSets can store any value, while WeakSets can only store the Set of objects. 2.WeakSet has no size property different from Set. 3.WeakSet does not have clear, keys, values, entries, forEach and other methods. 4.WeakSet is not iterative.
Let’s go back to the top and we’re on problem 100 and we’re on problem 200
205. List the set of methods available on WeakSet?
Below is a list of methods available on WeakSet,
1. Add (value) : append the given value to the new object of the WeakSet. 2. Delete (value) : delete the value from the WeakSet. 3. Has (value) : Return true if the value exists in the WeakSet, otherwise return false. 4. Length () : It returns the length of weakSetObject. Let’s see what all of the above methods do in an example,
var weakSetObject = new WeakSet(a);var firstObject = {};
var secondObject = {};
// add(value)
weakSetObject.add(firstObject);
weakSetObject.add(secondObject);
console.log(weakSetObject.has(firstObject)); //true
console.log(weakSetObject.length()); / / 2
weakSetObject.delete(secondObject);
Copy the codeLet’s go back to the top and we’re on problem 100 and we’re on problem 200
206. What is WeakMap?
A WeakMap object is a collection of key/value pairs where the key is weakly referenced. In this case, the key must be an object and the value can be any value. The syntax is as follows,
new WeakMap([iterable])
Copy the codeLet’s look at the following example to explain its behavior,
var ws = new WeakMap(a);var user = {};
ws.set(user);
ws.has(user); // true
ws.delete(user); // Delete the user from the map
ws.has(user); // false, user has been deleted
Copy the codeLet’s go back to the top and we’re on problem 100 and we’re on problem 200
207. What is the difference between a WeakMap and a Map?
The main difference is that the reference of key objects in Map is strong reference, while the reference of key objects in WeakMap is weak reference. That is, key objects in a WeakMap can be garbage collected if there are no other references.
The other difference is,
1.Maps can store any type of key, while WeakMaps can only store the set 2 of key objects. Different from Map, WeakMap does not have size property. 3.WeakMap does not have clear, keys, values, entries, forEach and other methods. 4.WeakMap cannot be iterated.
Let’s go back to the top and we’re on problem 100 and we’re on problem 200
208. List the set of methods available on WeakMap?
Below is a list of methods available on WeakMap,
1. Set (key, value) : Sets the key value in the WeakMap object. Return the WeakMap object. 2. Delete (key) : Deletes any value associated with the key. 3. Has (key) : Returns a Boolean value to assert whether a value is associated with a key in a WeakMap object. 4. Get (key) : Returns the value associated with the key. If not, undefined is returned. Let’s look at the capabilities of all of the above methods in one example,
var weakMapObject = new WeakMap(a);var firstObject = {};
var secondObject = {};
// set(key, value)
weakMapObject.set(firstObject, 'Haiyong');
weakMapObject.set(secondObject, 100);
console.log(weakMapObject.has(firstObject)); //true
console.log(weakMapObject.get(firstObject)); // Haiyong
weakMapObject.delete(secondObject);
Copy the codeLet’s go back to the top and we’re on problem 100 and we’re on problem 200
209. What is the purpose of Uneval?
Uneval () is a built-in function that creates a string representation of an object’s source code. It is a top-level function and is not associated with any object. Let’s take a look at the following example to learn more about what it does,
var a = 1;
uneval(a); // Returns a string containing 1
uneval(function user() {}); // returns "(function user(){})"
Copy the codeLet’s go back to the top and we’re on problem 100 and we’re on problem 200
210. How do you encode a URL?
The encodeURI() function is used to divide (, /? The full URI with special characters is encoded outside the: @ & = + $#) character.
var uri = 'https://mozilla.org/?x= ш е л л ы';
var encoded = encodeURI(uri);
console.log(encoded); // https://mozilla.org/?x=%D1%88%D0%B5%D0%BB%D0%BB%D1%8B
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211. How do you decode a URL?
The decodeURI() function is used to decode the uniform resource Identifier (URI) previously created by encodeURI().
var uri = 'https://mozilla.org/?x= ш е л л ы';
var encoded = encodeURI(uri);
console.log(encoded); // https://mozilla.org/?x=%D1%88%D0%B5%D0%BB%D0%BB%D1%8B
try {
console.log(decodeURI(encoded)); / / https://mozilla.org/?x= "ш е л л ы"
} catch(e) { // Catch ill-formed URIs
console.error(e);
}
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212. How do you print the content of a web page?
The window object provides a print() method that prints the contents of the current window. It opens a print dialog box that lets you choose between various print options. Let’s look at the use of the print method in an example.
<input type="button" value="Print" onclick="window.print()" />
Copy the codeNote: In most browsers, it will be blocked when the print dialog opens.
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213. What is the difference between uneval and eval?
The uneval function returns the source of the given object; The eval function, on the other hand, evaluates source code in different areas of memory. Let’s look at an example to clarify the difference,
var msg = uneval(function greeting() { return 'Hello, Good morning'; });
var greeting = eval(msg);
greeting(); // returns "Hello, Good morning"
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214. What is an anonymous function?
An anonymous function is a function without a name! Anonymous functions are usually assigned to variable names or used as callback functions. Syntax is as follows,
function (optionalParameters) {
//do something
}
const myFunction = function(){ // An anonymous function assigned to a variable
//do something
};
[1.2.3].map(function(element){ // The anonymous function is used as a callback function
//do something
});
Copy the codeLet’s look at the anonymous function above in an example,
var x = function (a, b) {return a * b};
var z = x(5.10);
console.log(z); / / 50
Copy the codeLet’s go back to the top and we’re on problem 100 and we’re on problem 200
215. What is the priority of local and global variables?
Local variables take precedence over global variables of the same name. Let’s look at this behavior in an example.
var msg = "Good morning";
function greeting() {
msg = "Good Evening";
console.log(msg);
}
greeting();
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216. What are javascript accessors?
ECMAScript 5 introduces javascript object accessors, or computed properties, through getters and setters. Getters use the get keyword, and Setters use the set keyword.
var user = {
firstName: "Hai".lastName : "Yong".language : "cn".get lang() {
return this.language;
}
set lang(lang) {
this.language = lang; }};console.log(user.lang); // Getter access lang as cn
user.lang = 'en';
console.log(user.lang); // setter 用于将 lang 设置为 en
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217. How do you define attributes on object constructors?
The object.defineProperty () static method is used to define new properties directly on an Object, or to modify existing properties on an Object, and return the Object. Let’s look at an example to see how attributes are defined,
const newObject = {};
Object.defineProperty(newObject, 'newProperty', {
value: 100.writable: false
});
console.log(newObject.newProperty); / / 100
newObject.newProperty = 200; // It throws an error in strict mode due to writable Settings
Copy the codeLet’s go back to the top and we’re on problem 100 and we’re on problem 200
218. What’s the difference between Get and defineProperty?
Unless you use classes, both have similar results. If you use get this property will be defined on the prototype of the Object, and if you use Object.defineProperty() it will be defined on the instance to which it is applied.
Let’s go back to the top and we’re on problem 100 and we’re on problem 200
219. What are the advantages of getters and setters?
Here’s a list of benefits of getters and setters,
1. They provide simpler syntax 2. They are used to define accessors in computed properties or JS. 3. Used to provide equivalence between properties and methods 4. They can provide better data quality 5. Useful for working behind the scenes with encapsulated logic.
Let’s go back to the top and we’re on problem 100 and we’re on problem 200
220. Can I add getters and setters using the defineProperty method?
Yes, you can add getters and setters using the object.defineProperty () method. For example, the following counter object uses increment, decrement, plus or minus properties,
var obj = {counter : 0};
// Define getters
Object.defineProperty(obj, "increment", {
get : function () {this.counter++;}
});
Object.defineProperty(obj, "decrement", {
get : function () {this.counter--;}
});
// Define setters
Object.defineProperty(obj, "add", {
set : function (value) {this.counter += value;}
});
Object.defineProperty(obj, "subtract", {
set : function (value) {this.counter -= value;}
});
obj.add = 10;
obj.subtract = 5;
console.log(obj.increment); / / 6
console.log(obj.decrement); / / 5
Copy the codeLet’s go back to the top and we’re on problem 100 and we’re on problem 200
221. What is the purpose of switch-case?
The switch case statement in JavaScript is used for decision purposes. In some cases, it is more convenient to use a switch case statement than an if-else statement. Syntax is as follows,
switch (expression)
{
case value1:
statement1;
break;
case value2:
statement2;
break; .case valueN:
statementN;
break;
default:
statementDefault;
}
Copy the codeThe multi-way branching statement above provides a simple way to dispatch execution to different parts of the code based on the value of the expression.
Let’s go back to the top and we’re on problem 100 and we’re on problem 200
222. What are the rules for using swtich Case?
Here is a list of rules to be aware of,
1. Expressions can be numeric or string types. 2. Expressions do not allow duplicate values. 3. The default statement is optional. If the expression passed to the switch does not match any case value, the statement in the default case is executed. 4. The break statement is used in switch to terminate a sequence of statements. 5. The break statement is optional. But if omitted, execution continues to the next case.
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223. What is a primitive data type?
A primitive data type is data that has an original value (no attributes or methods). There are seven raw data types.
1.string 2.number 3.boolean 4.null 5.undefined 6.bigint 7.symbol
Let’s go back to the top and we’re on problem 100 and we’re on problem 200
224. What are the different ways to access object properties?
There are three possible ways to access an object’s properties.
Dot notation: It uses dots to access properties
objectName.property
Copy the codeSquare bracket notation: It uses square brackets for property access
objectName["property"]
Copy the codeExpression notation: It uses expressions in square brackets
objectName[expression]
Copy the codeLet’s go back to the top and we’re on problem 100 and we’re on problem 200
225. What are function argument rules?
JavaScript functions follow the following argument rules,
1. The function defines data types that do not specify parameters. 2. Do not perform type checking on passed parameters. 3. Do not check the number of parameters received. That is, the following functions follow the above rules,
function functionName(parameter1, parameter2, parameter3) {
console.log(parameter1); / / 1
}
functionName(1);
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226. What is an error object?
An error object is a built-in error object that provides error information when an error occurs. It has two attributes: name and message. For example, the following function logs error details,
try {
greeting("Welcome");
}
catch(err) {
console.log(err.name + "<br>" + err.message);
}
Copy the codeLet’s go back to the top and we’re on problem 100 and we’re on problem 200
227. When you receive a grammar error
If you try to evaluate code with syntax errors, a SyntaxError is raised. For example, the missing function parameter reference below raises a syntax error
try {
eval("greeting('welcome)"); // Missing ' will produce an error
}
catch(err) {
console.log(err.name);
}
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228. What are the different error names for error objects?
The error object returns six different types of error names,
| Wrong name | instructions |
|---|---|
| EvalError | The eval() function failed |
| RangeError | Number out of range error |
| ReferenceError | An error caused by an illegal reference |
| SyntaxError | Errors caused by syntax errors |
| TypeError | Error caused by type error |
| URIError | Error due to encodeURI() |
229. What are the various statements in error handling?
The following is a list of statements used in error handling,
1. Try: this statement is used to test if a code block has errors. 2. Catch: this statement is used to handle errors. 3. 4. Finally: This statement is used to execute code after a try and catch, regardless of the result.
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230. What are the two types of loops in javascript?
Controlled Loops: In this type of loop, test conditions before entering the loop body. For example, For loops and While loops fall into this category.
Exit Controlled Loops: In this type of loop, test conditions are tested or evaluated at the end of the loop. That is, the body of the loop will execute at least once regardless of whether the test condition is true or false. For example, a do-while loop falls into this category.
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231. What is NodeJS?
Node.js is a server-side platform built on Chrome’s JavaScript runtime for easily building fast and extensible web applications. It is an event-based, non-blocking, asynchronous I/O runtime that uses Google’s V8 JavaScript engine and libuv library.
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232. What is an Intl object?
Intl objects are namespaces of the ECMAScript internationalization API that provide language-sensitive string comparisons, number formats, and date and time formats. It provides access to several constructors and language-sensitive functions.
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233. How do you perform language-specific date and time formatting?
You can use the Intl.dateTimeFormat object, which is the constructor of an object that enables language-sensitive date and time formats. Let’s look at this behavior with an example,
var date = new Date(Date.UTC(2019.07.07.3.0.0));
console.log(new Intl.DateTimeFormat('en-GB').format(date)); / / 07/08/2019
console.log(new Intl.DateTimeFormat('en-AU').format(date)); / / 07/08/2019
Copy the codeLet’s go back to the top and we’re on problem 100 and we’re on problem 200
234. What is an iterator?
An iterator is an object that defines a sequence and a return value on termination. It implements the iterator protocol using a next() method that returns an object with two attributes :(the next value in the sequence of values) and done (true if the last value in the sequence has been consumed).
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235. How does synchronous iteration work?
Synchronous iteration was introduced in ES6, which applies to the following set of components,
Iterable: It is an object that can be iterated through a method whose key is symbol. iterator. Iterator: This is an object returned by calling the [symbol.iterator]() iterable. This iterator object wraps the elements of each iteration in an object and returns them one by one through the next() method. IteratorResult: The object returned by the next() method. This object contains two properties; The value attribute contains an iteration element, and the done attribute determines whether the element is the last.
Let’s demonstrate synchronous iteration with the following array,
const iterable = ['one'.'two'.'three'];
const iterator = iterable[Symbol.iterator]();
console.log(iterator.next()); // { value: 'one', done: false }
console.log(iterator.next()); // { value: 'two', done: false }
console.log(iterator.next()); // { value: 'three', done: false }
console.log(iterator.next()); // { value: 'undefined, done: true }
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236. What is an event loop?
An event loop is a queue of callback functions. When an asynchronous function executes, the callback function is pushed to the queue. The JavaScript engine does not start processing the event loop until the asynchronous function has finished executing the code.
== Note == : Even if JavaScript is single-threaded, it allows Node.js to perform non-blocking I/O operations.
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237. What is the call stack?
The call stack is a data structure in the javascript interpreter that keeps track of function calls in your program. It has two main actions,
1. Every time you call a function to execute it, you are pushing it onto the stack. 2. Every time execution is complete, the function pops off the stack.
Let’s take an example of a state representation in chart format
function hungry() {
eatFruits();
}
function eatFruits() {
return "I'm eating fruits";
}
// Invoke the `hungry` function
hungry();
Copy the codeThe above code is handled in the call stack as follows,
1. Add the hungry() function to the call stack list and execute the code. 2. Add the eatFruits() function to the call stack list and execute the code. 3. EatFruits () Remove this function from our call stack list. 4. Hungry () removes this function from the call stack list because there are no more items.
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238. What is an event queue?
Temporal decoupling of the sending and processing of messages or events. In layman’s terms, a series of notifications or requests are stored in a queue in first-in, first-out order. When a notification is sent, the request is queued and returned. The system that processes the request later retrieves the request from the queue and processes it.
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239. What is a decorator?
A decorator is an expression that evaluates to a function and takes the target, name, and decorator descriptor as parameters. In addition, it can optionally return a decorator descriptor to install on the target object. Let’s define administrative decorators for user classes at design time,
function admin(isAdmin) {
return function(target) {
target.isAdmin = isAdmin;
}
}
@admin(true)
class User(a){}console.log(User.isAdmin); //true
@admin(false)
class User(a){}console.log(User.isAdmin); //false
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240. What are the properties of the Intl object?
The following is a list of properties available on Intl objects,
1.Collator: These are the objects that enable language-sensitive string comparisons. 2.DateTimeFormat: These are objects that enable language-sensitive date and time formats. ListFormat: these are language-sensitive ListFormat enabled objects. NumberFormat: Objects that enable language-sensitive number formats. 5.PluralRules: Objects that enable plural-sensitive formats and plural-language-specific rules. 6.RelativeTimeFormat: An object with language-sensitive relative time formats enabled.
Let’s go back to the top and we’re on problem 100 and we’re on problem 200
241. What is a unary operator?
The unary (+) operator is used to convert a variable to a number. If the variable cannot be converted, it will still become a number, but with a NaN value. Let’s look at this behavior in an action.
var x = "100";
var y = + x;
console.log(typeof x, typeof y); // string, number
var a = "Hello";
var b = + a;
console.log(typeof a, typeof b, b); // string, number, NaN
Copy the codeLet’s go back to the top and we’re on problem 100 and we’re on problem 200
242. How do I sort the elements in an array?
The sort() method is used to sort array elements in place and return the sorted array. Example usage is as follows,
var months = ["Aug"."Sep"."Jan"."June"];
months.sort();
console.log(months); // ["Aug", "Jan", "June", "Sep"]
Copy the codeLet’s go back to the top and we’re on problem 100 and we’re on problem 200
243. What is the purpose of compareFunction when sorting arrays?
CompareFunction is used to define the sort order. If omitted, the array elements are converted to strings, which are then sorted according to the Unicode code point value of each character. To take an example of the use of compareFunction,
let numbers = [1.2.5.3.4];
numbers.sort((a, b) = > b - a);
console.log(numbers); // [5, 4, 3, 2, 1]
Copy the codeLet’s go back to the top and we’re on problem 100 and we’re on problem 200
244. How do you reverse an array?
You can use the reverse() method to reverse elements in an array. This method is useful for sorting arrays in descending order. Let’s look at the use of the reverse() method in an example,
let numbers = [1.2.5.3.4];
numbers.sort((a, b) = > b - a);
numbers.reverse();
console.log(numbers); // [1, 2, 3, 4,5]
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245. How do you find the minimum and maximum values in an array?
You can use the math.min and math.max methods on array variables to find the smallest and largest elements in an array. Let’s create two functions to find the minimum and maximum values in the array,
var marks = [50.20.70.60.45.30];
function findMin(arr) {
return Math.min.apply(null, arr);
}
function findMax(arr) {
return Math.max.apply(null, arr);
}
console.log(findMin(marks));
console.log(findMax(marks));
Copy the codeLet’s go back to the top and we’re on problem 100 and we’re on problem 200
246. How do I find min and Max values without math functions?
You can write functions that loop through groups of numbers, comparing each value to a minimum or maximum to find the minimum and maximum. Let’s create these functions to find minimums and maximums,
var marks = [50.20.70.60.45.30];
function findMin(arr) {
var length = arr.length
var min = Infinity;
while (length--) {
if(arr[length] < min) { min = arr[len]; }}return min;
}
function findMax(arr) {
var length = arr.length
var max = -Infinity;
while (len--) {
if(arr[length] > max) { max = arr[length]; }}return max;
}
console.log(findMin(marks));
console.log(findMax(marks));
Copy the codeLet’s go back to the top and we’re on problem 100 and we’re on problem 200
247. What is a null statement and its purpose?
The empty statement is a semicolon (;) , indicating that no statement will be executed, even if JavaScript syntax requires one. Since empty statements have no actions, you might think they are rarely used, but they can sometimes be useful when you want to create loops with empty bodies. For example, you can initialize an array with a zero value, as shown below,
// Initialize an array a
for(int i=0; i < a.length; a[i++] = 0);Copy the codeLet’s go back to the top and we’re on problem 100 and we’re on problem 200
248. How to obtain module metadata?
You can expose context-specific metadata to JavaScript modules using import.meta as an object with meta-attributes. It contains information about the current module, such as the URL of the module. In a browser, you might get metadata that is different from NodeJS.
<script type="module" src="welcome-module.js"></script>
console.log(import.meta); // { url: "file:///home/user/welcome-module.js"
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249. What is a comma operator?
The comma operator evaluates each of its operands from left to right and returns the value of the last operand. This is completely different from the use of commas in array, object, and function arguments and arguments. For example, a numeric expression can be used as follows,
var x = 1;
x = (x++, x);
console.log(x); / / 2
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250. What are the advantages of the comma operator?
It is typically used to include multiple expressions where a single expression is needed. One common use of this comma operator is to provide multiple arguments in a for loop. For example, the following for loop uses the comma operator to use multiple expressions in a single place,
for (var a = 0, b =10; a <= 10; a++, b--)
Copy the codeYou can also use the comma operator in a return statement, which is processed before the return.
function myFunction() {
var a = 1;
return (a += 10, a); / / 11
}
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251. What is typescript?
TypeScript is a typed superset of JavaScript created by Microsoft that adds optional types, classes, async/await, and many other features and compiles to pure JavaScript. Angular is built entirely in TypeScript and is used as the primary language. You can install it globally as
npm install -g typescript
Copy the codeLet’s look at a simple example of TypeScript usage.
function greeting(name: string) :string {
return "Hello, " + name;
}
let user = "Sudheer";
console.log(greeting(user));
Copy the codeThe greeting method only allows string types as arguments.
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252. What’s the difference between javascript and typescript?
Here’s a list of the differences between javascript and typescript
| function | typescript | javascript |
|---|---|---|
| Language paradigm | Object-oriented programming language | Scripting language |
| Typing support | Static typing support | It has dynamic typing |
| The module | support | Does not support |
| interface | It has the concept of interfaces | Interface not supported |
| Optional parameters | The function supports optional arguments | Optional arguments to functions are not supported |
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253. What are typescript’s advantages over javascript?
Here are some advantages of typescript over javascript,
1.TypeScript only finds compile-time errors at development time and ensures that runtime errors are minimized. Javascript is an interpreted language. 2.TypeScript is strongly typed or supports static typing, allowing type correctness to be checked at compile time. This is not available in javascript. 3. The TypeScript compiler compiles.ts files to ES3, ES4, and ES5, unlike the ES6 features of JavaScript that some browsers may not support.
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254. What is an object initializer?
An object initializer is an expression describing the initialization of an object. The syntax for this expression is a comma-separated list of zero or more pairs of attribute names and object associated values, enclosed in curly braces ({}). This is also called literal notation. It’s one of the ways to create an object.
var initObject = {a: 'Haiyong'.b: 50.c: {}};
console.log(initObject.a); // Haiyong
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255. What is a constructor?
Constructor methods are special methods used to create and initialize objects created in a class. If no constructor method is specified, the default constructor is used. An example use of the constructor is as follows,
class Employee {
constructor() {
this.name = "Haiyong"; }}var employeeObject = new Employee();
console.log(employeeObject.name); // Haiyong
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256. What happens if a constructor is written more than once in a class?
A “constructor” in a class is a special method that can only be defined once in a class. That is, if you write a constructor method multiple times in a class, it will throw a SyntaxError.
class Employee {
constructor() {
this.name = "Haiyong";
}
constructor() { // Uncaught SyntaxError: A class may only have one constructor
this.age = 30; }}var employeeObject = new Employee();
console.log(employeeObject.name);
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257. How do I call the constructor of a parent class?
You can use the super keyword to call the constructor of the parent class. Remember that super() must be called before using the “this” reference. Otherwise, reference errors will result. Let’s use it,
class Square extends Rectangle {
constructor(length) {
super(length, length);
this.name = 'Square';
}
get area() {
return this.width * this.height;
}
set area(value) {
this.area = value; }}Copy the codeGo back to the top and we’re on problem 250
258. How do you get the prototype of an object?
You can use the object.getProtoTypeof (obj) method to return the prototype of a specified Object. The value of the internal Prototype property. If there is no inherited attribute, null returns the value.
const newPrototype = {};
const newObject = Object.create(newPrototype);
console.log(Object.getPrototypeOf(newObject) === newPrototype); // true
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259. What happens if I pass a string type to the getPrototype method?
In ES5, if the obj argument is not an object, it raises TypeError. In ES2015, this parameter is cast to Object.
// ES5
Object.getPrototypeOf('CSDN'); // TypeError: "CSDN" is not an object
// ES2015
Object.getPrototypeOf('CSDN'); // String.prototype
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260. How do I set the prototype of one object to another?
You can use methods that set the Prototype (that is, the internal Prototype property) of the specified Object Object.setProtoTypeof () to another Object or null. For example, if you wanted to set the prototype of a square object to a rectangular object, you would look like this,
Object.setPrototypeOf(Square.prototype, Rectangle.prototype);
Object.setPrototypeOf({}, null);
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261. How do you check if an object is extensible?
The object.isextensible () method is used to determine whether an Object isExtensible. That is, whether it can add new attributes.
const newObject = {};
console.log(Object.isExtensible(newObject)); //true
Copy the code== Note == : By default, all objects are extensible. That is, new attributes can be added or modified.
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262. How do I prevent object expansion?
The object.preventExtensions () method is used to prevent new properties from being added to the Object. In other words, it prevents future extensions to objects. Let’s look at how this property is used,
const newObject = {};
Object.preventExtensions(newObject); // Not extensible
try {
Object.defineProperty(newObject, 'newProperty', { // Add a new attribute
value: 100
});
} catch (e) {
console.log(e); // Type error: attribute newProperty cannot be defined, object is not extensible
}
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263. What are the different ways to make an object unextensible?
You can mark an object as unextensible in three ways,
1.Object.preventExtensions 2.Object.seal 3.Object.freeze
var newObject = {};
Object.preventExtensions(newObject); // Prevent objects from being unextensible
Object.isExtensible(newObject); // false
var sealedObject = Object.seal({}); // The sealed object cannot be extended
Object.isExtensible(sealedObject); // false
var frozenObject = Object.freeze({}); // Frozen objects are not extensible
Object.isExtensible(frozenObject); // false
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264. How can I define multiple attributes on an object?
The object.defineProperties () method is used to define new properties directly on an Object or modify existing properties and return the Object. Let’s define multiple properties on an empty object,
const newObject = {};
Object.defineProperties(newObject, {
newProperty1: {
value: 'Haiyong'.writable: true
},
newProperty2: {}});Copy the codeGo back to the top and we’re on problem 250
265. What is MEAN in JavaScript?
The MEAN (MongoDB, Express, AngularJS, and Node.js) stack is the most popular open source JavaScript software technology stack for building dynamic Web applications, You can write the server-side and client-side parts of your Web project entirely in JavaScript.
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266. What is confusion in javascript?
Obfuscation is the deliberate act of creating obfuscated javascript code (that is, source code or machine code) that is difficult for humans to understand. It is similar to encryption, but the machine can understand the code and execute it. Let’s look at the following function before we get confused,
function greeting() {
console.log('Hello, welcome to JS world');
}
Copy the codeAfter code obfuscation, it appears as follows,
eval(function(p,a,c,k,e,d){e=function(c){return c};if(!' '.replace(/ ^ /.String)
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267. Why confusion?
Here are a few reasons for confusion,
1. Code size will be reduced. So the data transfer between the server and the client is very fast. It hides business logic from the outside world and protects code from others. 3. Reverse engineering is difficult. Download times will be reduced
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268. What is Minification?
Zoom is the process of removing all unnecessary characters (removing Spaces), and variables are renamed without changing their function. This is also a kind of confusion.
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269. What are the benefits of shrinking?
In general, it is recommended to use minimization in cases of heavy traffic and resource-intensive requirements. It reduces file size with the following benefits,
1. Reduce web page loading time 2. Save bandwidth usage
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270. What is the difference between obfuscation and encryption?
Here are the main differences between obfuscation and encryption,
| features | confusion | encryption |
|---|---|---|
| define | Change the form of any data in any other form | Use a key to change the form of information to an unreadable format |
| The key to decoding | Can be decoded without any key | It is required |
| Target data format | It will be converted into complex forms | Convert to an unreadable format |
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271. What are the commonly used miniaturization tools?
There are many online/offline tools for miniaturizing javascript files,
- Google’s Closure compiler
- jsmin
- javascript-minifier.com
- Prettydiff.com
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272. How do you perform form validation using javascript?
JavaScript can be used to perform HTML form validation. For example, if the form field is empty, the function needs to be notified and returns false to prevent the form from being submitted. Let’s perform the user login in HTML,
<form name="myForm" onsubmit="return validateForm()" method="post">
User name: <input type="text" name="uname">
<input type="submit" value="Submit">
</form>
Copy the codeUser login verification is as follows,
function validateForm() {
var x = document.forms["myForm"] ["uname"].value;
if (x == "") {
alert("The username shouldn't be empty");
return false; }}Copy the codeGo back to the top and we’re on problem 250
273. How do I perform form validation without javascript?
You can perform HTML form validation automatically without using javascript. Enable validation by applying the Required attribute to prevent submission of the form when the input is empty.
<form method="post">
<input type="text" name="uname" required>
<input type="submit" value="Submit">
</form>
Copy the code== Note == : Automatic form validation does not work in Internet Explorer 9 or earlier.
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274. What are the DOM methods available for constraint validation?
The following DOM methods can be used to constrain validation of invalid input,
CheckValidity () : Returns true if the input element contains valid data. 2. SetCustomValidity () : Used to set the validationMessage property of the input element. Let’s use a user login form with DOM authentication
function myFunction() {
var userName = document.getElementById("uname");
if(! userName.checkValidity()) {document.getElementById("message").innerHTML = userName.validationMessage;
} else {
document.getElementById("message").innerHTML = "Entered a valid username"; }}Copy the codeGo back to the top and we’re on problem 250
275. What are the constraints available to validate DOM attributes?
Here is a list of the DOM attributes that are available for constraint validation,
1. Validity: It provides a list of Boolean attributes related to the validity of the input element. 2. ValidationMessage: Displays messages when validity is false. 3. WillValidate: Indicates whether the input element will be validated.
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276. What is a validity attribute list?
The validity attributes of an input element provide a set of attributes related to data validity.
1. CustomError: Returns true if a custom validity message is set. 2. PatternMismatch: Return true if the value of the element does not match its schema attribute. 3. RangeOverflow: returns true if the value of an element is greater than its Max attribute. RangeUnderflow: Returns true if the value of an element is less than its min attribute. 5. StepMismatch: Return true if the element value is invalid based on the step attribute. TooLong: Returns true if the value of an element exceeds its maxLength attribute. TypeMismatch: Returns true if the value of the element is invalid based on the type attribute. ValueMissing: Returns true if the element with the required attribute has no value. 9. Valid: Returns true if the element value is valid.
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277. Example of using the rangeOverflow property?
The rangeOverflow property returns true if the value of an element is greater than its Max property. For example, if the value is greater than 100, the following form reference raises an error,
<input id="age" type="number" max="100">
<button onclick="myOverflowFunction()">OK</button>
Copy the code function myOverflowFunction() {
if (document.getElementById("age").validity.rangeOverflow) {
alert("The mentioned age is not allowed"); }}Copy the codeGo back to the top and we’re on problem 250
278. Is enumeration provided in javascript?
No, javascript itself does not support enumerations. But there are different kinds of solutions that can simulate them, even though they may not provide exact equivalents. For example, you can use freezes or seals on objects,
var DaysEnum = Object.freeze({"monday":1."tuesday":2."wednesday":3. })Copy the codeGo back to the top and we’re on problem 250
279. What is enumeration?
Enumeration is a type that restricts a variable to a single value from a predefined set of constants. JavaScript doesn’t have enumerations, but typescript does have built-in support for them.
enum Color {
RED, GREEN, BLUE
}
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280. How do you list all the attributes of an object?
You can use this Object. GetOwnPropertyNames () method returns directly find all of the attributes in the given Object array. Let’s use it in an example,
const newObject = {
a: 1.b: 2.c: 3
};
console.log(Object.getOwnPropertyNames(newObject)); ["a"."b"."c"]
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281. How do I get the property descriptor of an object?
You can use Object. GetOwnPropertyDescriptors () returns a given Object method of all his property descriptors. An example use of this method is as follows,
const newObject = {
a: 1.b: 2.c: 3
};
const descriptorsObject = Object.getOwnPropertyDescriptors(newObject);
console.log(descriptorsObject.a.writable); //true
console.log(descriptorsObject.a.configurable); //true
console.log(descriptorsObject.a.enumerable); //true
console.log(descriptorsObject.a.value); / / 1
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282. What properties do attribute descriptors provide?
Property descriptors are records that have the following properties
Writable: disables any additional information that can be changed without any additional control. If the type of the property descriptor can be changed and the property can be deleted from any additional object, the 64x works without any additional control. 4. Enumerable: Determine if the property appears when enumerating a property on the corresponding object. Set: a function that acts as a setter for properties. 6. Get: a function that acts as a getter for properties
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283. How do you extend classes?
The extends keyword is used in a class declaration/expression to create a class that is a subclass of another class. It can be used to subclass custom classes as well as built-in objects. Syntax is as follows,
class ChildClass extends ParentClass {... }Copy the codeLet’s take the Square subclass of Polygon’s parent class,
class Square extends Rectangle {
constructor(length) {
super(length, length);
this.name = 'Square';
}
get area() {
return this.width * this.height;
}
set area(value) {
this.area = value; }}Copy the codeGo back to the top and we’re on problem 250
284. How do I change the URL without reloading the page?
The window.location.url property will help modify the URL, but it will reload the page. HTML5 introduces history.pushstate () and history.replacestate () methods, which allow you to add and modify history entries, respectively. For example, you can use pushState as follows,
window.history.pushState('page2'.'Title'.'/page2.html');
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285. How do I check if an array contains a specific value?
The Array#includes() method is used to determine whether an array contains a particular value in its entries by returning true or false. Let’s look at an example of finding elements (numbers and strings) in an array.
var numericArray = [1.2.3.4];
console.log(numericArray.includes(3)); // true
var stringArray = ['green'.'yellow'.'blue'];
console.log(stringArray.includes('blue')); //true
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286. How do you compare scalar arrays?
You can compare two scalar arrays (directly using === for comparison) using length and the array’s every method. The combination of these expressions gives the desired result,
const arrayFirst = [1.2.3.4.5];
const arraySecond = [1.2.3.4.5];
console.log(arrayFirst.length === arraySecond.length && arrayFirst.every((value, index) = > value === arraySecond[index])); // true
Copy the codeIf you want to compare arrays regardless of order, then you should sort them first,
const arrayFirst = [2.3.1.4.5];
const arraySecond = [1.2.3.4.5];
console.log(arrayFirst.length === arraySecond.length && arrayFirst.sort().every((value, index) = > value === arraySecond[index])); //true
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287. How do I get values from get parameters?
The new URL() object accepts the URL string searchParams, whose properties can be used to access the fetch parameters. Keep in mind that you may need to use polyfill or window.location to access urls in older browsers, including Internet Explorer.
let urlString = "http://www.some-domain.com/about.html?x=1&y=2&z=3"; //window.location.href
let url = new URL(urlString);
let parameterZ = url.searchParams.get("z");
console.log(parameterZ); / / 3
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288. How do you print numbers using commas as thousands separators?
You can use the Number. The prototype. ToLocaleString () method returns the string with language sensitive said, for example, a one thousand – bit separator, currency, etc.
function convertToThousandFormat(x){
return x.toLocaleString(); / / 12345679
}
console.log(convertToThousandFormat(12345.6789));
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289. What is the difference between Java and javascript?
Both are completely unrelated programming languages, and there is no relationship between them. Java is statically typed, compiled, and runs on its own VM. Javascript, on the other hand, is dynamically typed, interpreted, and run in a browser and nodeJS environment. Let’s look at the main differences in table formats
| features | java | JavaScript |
|---|---|---|
| type | It is a strongly typed language | It is a dynamically typed language |
| paradigm | Object-oriented programming | Prototype-based programming |
| The scope of | Block scope | Function scope |
| concurrent | Based on the thread | Based on the event |
| memory | Use more memory | Use less memory. So it will be used for web pages |
Specific differences between reference: haiyong.blog.csdn.net/article/det…
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290. Does javascript support namespaces?
JavaScript does not support namespaces by default. Therefore, if you create any element (function, method, object, variable), it becomes global and pollutes the global namespace. Let’s take an example of defining two functions that do not have any namespaces
function func1() {
console.log("That's the first definition.");
}
function func1() {
console.log("That's the second definition.");
}
func1(); // This is the second definition
Copy the codeIt always calls the second function definition. In this case, namespace resolves name conflicts.
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291. How do you declare namespaces?
Although JavaScript lacks namespaces, we can create namespaces using Objects, IIFE.
Use object literals: Let’s wrap variables and functions in object literals that act as namespaces. You can then access them using object notation
var namespaceOne = {
function func1() {
console.log("This is a first definition"); }}var namespaceTwo = {
function func1() {
console.log("This is a second definition");
}
}
namespaceOne.func1(); // This is a first definition
namespaceTwo.func1(); // This is a second definition
Copy the codeUsing IIFE (Call function expressions now) : The outer pair of parentheses of IIFE creates a local scope for all code inside it and makes the anonymous function a function expression. Therefore, you can create the same function in two different function expressions to act as a namespace.
(function() {
function fun1(){
console.log("That's the first definition."); } fun1(); } ()); (function() {
function fun1(){
console.log("That's the second definition."); } fun1(); } ());Copy the codeUse block and LET /const declarations: In ECMAScript 6, you can simply use block and LET declarations to limit the scope of variables to blocks.
{
let myFunction= function fun1(){
console.log("That's the first definition.");
}
myFunction();
}
//myFunction(): ReferenceError: myFunction undefined.
{
let myFunction= function fun1(){
console.log("That's the second definition.");
}
myFunction();
}
//myFunction(): ReferenceError: myFunction undefined.
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292. How do I call javascript code in iframe from the parent page?
You initially need to use document.getelementby or access iFrame window.frames. After the contentWindowiFrame property provides access to targetFunction
document.getElementById('targetFrame').contentWindow.targetFunction();
window.frames[0].frameElement.contentWindow.targetFunction(); // Accessing iframe in this way may not work with the latest versions of Chrome and Firefox
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293. How do I get the time zone offset from the date?
You can use the getTimezoneOffset date object method. This method returns the time zone difference in minutes from the current locale (host system Settings) to UTC
var offset = new Date().getTimezoneOffset();
console.log(offset); / / - 480
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294. How do I load CSS and JS files dynamically?
You can create link and script elements in the DOM and append them as children to the head tag. Let’s create a function to add script and style resources, as shown below,
function loadAssets(filename, filetype) {
if (filetype == "css") { // External CSS file
var fileReference = document.createElement("link")
fileReference.setAttribute("rel"."stylesheet");
fileReference.setAttribute("type"."text/css");
fileReference.setAttribute("href", filename);
} else if (filetype == "js") { // External JavaScript file
var fileReference = document.createElement('script');
fileReference.setAttribute("type"."text/javascript");
fileReference.setAttribute("src", filename);
}
if (typeoffileReference ! ="undefined")
document.getElementsByTagName("head") [0].appendChild(fileReference)
}
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295. What are the different ways to find HTML elements in the DOM?
If you want to access any element in an HTML page, you need to start by accessing the document object. You can later find HTML elements using any of the following methods,
1. The document. The getElementById (id) : by id lookup element (2) the document. The getElementsByTagName (name) : Through the element tag name lookup 3. Document. GetElementsByClassName (name) : element via the name of the class
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296. What is jQuery?
JQuery is a popular cross-browser JavaScript library that provides document Object Model (DOM) traversal, event handling, animation, and AJAX interaction by minimizing differences between browsers. It is known for its “write less, do more” philosophy. For example, you can use jQuery to display a welcome message when the page loads, as shown below,
$(document).ready(function(){ // It selects the document and applies it when the page loads
alert('Welcome to jQuery world');
});
Copy the code== Note == : You can download it from the official jquery website or install it from a CDN (such as Google).
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297. What is the V8 JavaScript engine?
V8 is an open source, high-performance JavaScript engine for Google Chrome, written in C++. It is also used in Node.js projects. It implements ECMAScript and WebAssembly and runs on Windows 7 or later, macOS 10.12+, and Linux systems using X64, IA-32, ARM, or MIPS processors.
== note == : this can be run independently or embedded in any C++ application.
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298. Why do we call javascript a dynamic language?
JavaScript is a loosely typed or dynamic language because variables in JavaScript are not directly associated with any particular value type, and any variable can assign/reassign values of all types.
let age = 50; // age is a number now
age = 'old'; // age is a string now
age = true; // age is a boolean
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299. What is the air freight operator?
The void operator evaluates the given expression and returns undefined (that is, no value). Syntax is as follows,
void (expression)
void expression
Copy the codeLet’s display a message without any redirection or reload
<a href="javascript:void(alert('Welcome to JS world'))">Click here to see a message</a>
Copy the code== Note that == : This operator is usually used to get an undefined raw value, using “void(0)”.
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300. How do I set cursor wait?
You can use the property “cursor” to set the cursor to wait in JavaScript. Let’s use the following function to perform this behavior when the page loads.
function myFunction() {
window.document.body.style.cursor = "wait";
}
Copy the codeThis function is called when the page loads
<body onload="myFunction()">
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