Dedicated to you on your way to job hunting (2)

Reprinted from: http://www.jianshu.com/p/87fa2c21039a

Note: In fact, in the real interview, the interviewer often uses the routine from difficult to easy, that JS and jQuery is the most important. As well as some questions about the project and the technology used.

JavaScript

What is a closure and why do you use it?

A closure is a function that can access the variables of an external function.

function aa(x){ var num=1; function bb(y){ console.log(x+y+(++num)); }}Copy the code

A bb function of a aa function is a closure. A BB function can take local variables and arguments of a AA function. A typical closure would look like this, with a function defined in a function as its return value

function aa(x){
       var num=1;
      function bb(y){
          console.log(x+y+(++num));
     }
return bb;
}Copy the code

Another function of closures is to isolate scopes, as shown in the following code

for(var i=0; i<2; i++){ setTimeout(function(){ console.log(i); }, 0); }Copy the code

The result of this code is 2,2 instead of 0,1, because by the time the for loop comes out and setTimeout is executed, the value of I has changed to 2.

Talk about understanding This object

Event is? What’s the difference between IE and Firefox’s event mechanism? How do I stop bubbling?

1. An action we take on a web page (some actions correspond to multiple events). For example, when we click a button an event is generated. Is behavior that can be detected by JavaScript.
2. Event handling mechanism: IE is event bubbling, Firefox is event capture;
3. ev.stopPropagation();

JavaScript scope and scope chain?

var t=4; function aa(){ var num1=12; funciton bb(){ var num2=34; console.log(t+" "+num1+" "+num2); }}Copy the code

“Use strict”; What does that mean? What are the advantages and disadvantages of using it?

ECMAscript 5 adds a second mode: “Strict mode”. As the name suggests, this mode makes Javascript run under more stringent conditions. The main objectives of the Strict Mode are as follows:

Advantages:

1. Eliminate some unreasonable and inaccurate Javascript syntax, reduce some weird behavior;
2. Eliminate some unsafe code operation, to ensure the safety of code operation;
3. Improve the efficiency of the compiler, increase the running speed;
4. Set the stage for future versions of Javascript. Note: After testing IE6,7,8, and 9 do not support strict mode.

Disadvantages:

JS on websites are now compressed, some files are in strict mode and others are not. These files are merged into the middle of the file, not only does it not indicate the strict mode, but it wastes bytes after compression.

What exactly does the new operator do?

var obj  = {};
obj.__proto__ = Base.prototype;
Base.call(obj);Copy the code

In Javascript, there’s a function that performs an object lookup that never looks up the prototype. What’s that function?

hasOwnProperty()

What do you know about JSON?

What are the ways to lazily load JS?

Defer and Async, create the DOM dynamically (most used), and load JS asynchronously on demand

What is Ajax? What are the pros and cons?

How to solve cross-domain problems?

Jsonp (using the cross-domain capability of script tags) cross-domain, WebSocket (a new feature of HTML5, a new protocol) cross-domain, set up proxy servers (servers that request data for us from servers of different sources), CORS (cross-source resource sharing, Cross Origin Resource Sharing), IFrame across domains, postMessage(pages containing IFrame pass messages to iframe)

How do you do modularity? Execute the function immediately without exposing private members

Var module1 = (function(){var _count = 0; Var m1 = function(){//... }; Var m2 = function(){//... }; Return {m1: m1, m2: m2}; }) ();Copy the code

What are the methods of asynchronous loading?

(1) defer, which only supports IE (2) async: false (true by default); (3) Create script and insert it into DOM. After loading, callBack

The difference between documen.write and innerHTML

The difference between.call() and.apply()?

Call (sub,3,1) == add(3,1); alert(4); Note: functions in js are actually objects, and Function names are references to Function objects.

function add(a,b) { alert(a+b); } function sub(a,b) { alert(a-b); } the add. Call (sub, 3, 1);Copy the code

What is the difference between Jquery and Jquery UI?

How to convert an array to a JSON string in jquery, and then back again?

JQuery does not provide this functionality, so you need to write two jQuery extensions first:

$.fn.stringifyArray = function(array) {
 return JSON.stringify(array)
}

$.fn.parseArray = function(array) {
 return JSON.parse(array)
} 

然后调用：
$("").stringifyArray(array)Copy the code

Optimization for jQuery?

Variable declaration enhancement in JavaScript?

Variables declared by var in a function are preloaded before the function body is executed

var num=1;
function aa(){
  console.log(num);     //undeifned
  var num=2; 
  console.log(num);  //2
}
aa();Copy the code

(num = aa, num = aa, num = aa, num = aa, num = aa, num = aa, num = aa, num = aa, num = aa, num = aa, num = aa

function aa(){ var num; console.log(num); //undefined num=2; console.log(num); / / 2}Copy the code

Which operations cause memory leaks?

How do I determine whether the script is currently running in a browser or node environment? (ali)

Check whether the Global object is window. If not, the script is not running in the browser

Of course, in an interview, in addition to knowing these basic knowledge, many times you need to know your work experience, do you have? Here are some of the problems, solutions, and thinking patterns in work and projects

Other problems (some of the problems I have encountered and summarized, for your reference)

• What is the most difficult technical problem you have encountered? How did you solve it?
• What libraries are commonly used? Common front-end development tools? What applications or components have you developed?
• How to reconstruct the page?
• List the features that make Internet Explorer different from other browsers.
• 99% of websites need to be refactored. What book is that?
• What is graceful degradation and progressive enhancement?
• How do WEB applications actively push Data from the server to the client?
• What is your role in your current team and what is your obvious impact?
• What other technologies do you know besides the front end? What is your greatest skill?
• How to design a burst large-scale concurrency architecture?
• What do you think is a Full Stack developer?
• Can you tell me one of your favorite works?
• What are your strengths? What are the disadvantages?
• How do you manage the front end team?
• What are you studying recently? Can you tell me where you see yourself in the next three to five years?

What are the advantages and disadvantages of Node?

What performance optimization methods do you have? (See Yahoo’s 14 Performance Optimization Principles)

(1) reduce the number of HTTP requests: CSS Sprites, JS, CSS source compression, image size control appropriate; Web Gzip, CDN hosting, data cache, picture server. (2) Front-end template JS+ data to reduce bandwidth waste caused by HTML tags. Front-end variables are used to save AJAX request results, so local variables are not required for each operation to reduce the number of requests. (3) Use innerHTML instead of DOM operations to reduce the number of DOM operations and optimize javascript performance. (4) Set className instead of style when there are many styles to set. (5) Use less global variables and cache the results of DOM node search. Reduce I/O read operations. Avoid using CSS Expression, also known as Dynamic properties. (7) Image preloading, style sheets at the top, scripts at the bottom with time stamps. (8) Avoid using tables in the main layout of a page. Tables are not displayed until the content is fully loaded, which is slower than div+ CSS layout.

What are the HTTP status codes? What does it mean to represent separately?

100-199 Indicates the actions that the client should take. 200-299 Indicates that the request is successful. 300-399 Is used for files that have been moved and is often included in the location header to specify new address information. 400-499 Indicates errors on the client. The current request cannot be understood by the server. 401 The current request requires the user to verify that the 403 server understands the request but refuses to execute it. 500-599 is used to support server errors. 503 -- Service unavailableCopy the code

What happens when a page is loaded and displayed from the time the URL is entered? (The more detailed the process, the better)

• Find the browser cache
• DNS resolves, searches for the IP address corresponding to the domain name, redirects (301), and sends the second GET request
• The HTTP session is performed
• Client sends headers (request headers)
• Server feedback header (response header)
• The HTML document starts to download
• A document tree is created to specify the MIME type of the file required by the tag request
• File display {browser side of the work is roughly divided into the following steps: load: according to the URL for domain name resolution, to the server to initiate a request, receive files (HTML, JS, CSS, images, etc.). Parsing: Parses the loaded resources (HTML, JS, CSS, etc.) and suggests the corresponding internal data structures (such as HTML DOM tree, JS property sheets, CSS style rules, etc.)}

What development tools do you use and why?

How do you understand the position of front-end interface engineer? What are its prospects?

• The front end is the programmer closest to the user, closer than the back end, database, product manager, operations, and security.
  • Interface interaction
  • Improve user experience
  • With Node.js, the front end can do some things on the server side
• The front end is the programmer closest to the user, and the power of the front end is to evolve a product from a 90 to a 100 or better,
• Participated in the project, completed the effect drawing quickly and with high quality, accurate to 1px;
• Communicate with team members, UI design, product manager;
• Good page structure, page reconstruction and user experience;
• Hack, compatible, write beautiful code format;
• Server optimization, embrace the latest front-end technology.

How do you manage your projects?

What are the most popular things these days? What websites do you visit?

Node.js, MongoDB, NPM, MVVM, MEAN, three.js, Angular, React, Vue Github, Zhihu, MDN, ES6, Bootstrap, git

Mobile terminal (Android IOS) how to do a good user experience?