Hello, today for everyone children’s shoes to share is JS, quickly take out a small notebook to write down
First Javascript
Function:
- Form dynamic verification (password strength detection)
- Web page special effects
- Server-side development (Node.js)
- Desktop Program (Electron)
- The App (Cordova)
- Control Hardware – Internet of Things (Ruff)
- Game Development (Cocos2D-JS)
HTML/CSS/JS
HTML/CSS Markup Language – Description class language
- HTML determines the structure and content of a web page, the equivalent of the human body
- CSS determines how a web page will look to the user, much like dressing or putting on makeup
JS scripting language – programming class language
Industrial business logic and page control (decision function), equivalent to various human actions
An introduction to browser JS execution
- Rendering engine: used to parse HTML and CSS, commonly known as kernel, such as Blink in Chrome, older versions of WebKit
- JS engine: also called JS interpreter. It is used to read Javascript code from a web page, process it and run it, such as Chrome V8
The browser itself does not execute JS code, but rather uses a built-in JavaScript engine (interpreter) to execute JS code. The JS engine interprets each line of source code line by line (translated into machine language) and then executes it by computer, so JavaScript language is classified as a scripting language that performs interpretive execution
The composition of JS
ECMAScript: JavaScript syntax
DOM: Page document object model
BOM: Browser object model
ECMAScript defines the programming syntax and basic core knowledge of JS. It is an industry standard of JS syntax that all browser manufacturers comply with
DOM – Document Object Model (DOM) : The DOCUMENT Object Model (DOM) is a standard programming interface recommended by the W3C for dealing with extensible Markup Language (XML). DOM provides interfaces for manipulating various elements on a page (size, location, color, etc.)
BOM – Browser object model: It provides a content-independent object structure that can interact with the browser window to manipulate the browser window, such as pop-ups, controlling browser jumps, obtaining resolution, etc
At the beginning of js experience
Js has three writing positions: inline, inline, and external
Note that we recommend double quotes in HTML and single quotes in JS
Alert (‘ hymn of valor ‘);
Note: No code can be written between script tags that reference external JS files
Js annotation
Input output statement
Js variable
Overview: What are variables
In plain English: A variable is a box of things
Popular: A variable is a container for storing data, which can be retrieved by its name and even modified
Declare a variable
// Declare the variable var age; // Declare a variable named age
Var is a JS keyword used to declare variables. After the keyword is used to declare variables, the computer automatically allocates memory space for the variables
Age is the name of the variable defined by the programmer
The assignment
age = 10; // Assign the age variable to 10
= is used to assign the value on the right to the variable space on the left
A variable value is a value that the programmer saves into the variable space
Initialization of a variable
var age = 18; // declare the variable and assign it to 18
Declaring a variable and assigning a value is called initialization of the variable
Use cases for variables
- An input box pops up, prompting the user to enter a name
- A dialog box pops up with the name the user just entered
Variable syntax extension
To update the variable
When a variable is reassigned, its original value is overwritten, and the value of the variable is the last value assigned
var age = 18; age = 81; // The final result is 81, because 18 is covered
Declare multiple variables at the same time
To declare multiple variables at the same time, write only one var and separate the variable names with commas (,)
var age = 10, name = ‘xx’, sex = 1;
Declare variable special case
Example: Swap the values of two variables
The data type of a variable
JavaScript is a weakly typed or dynamic language, which means that the type of a variable is not declared in advance, but is determined automatically as the program runs
var age = 10; Var areYouOk = ‘yes ‘; // This is a string
When the code runs, the data type of the variable is determined by the JS engine according to the data type of the value of the variable on the right of =. After running, the data type of the variable is determined.
JavaScript has dynamic typing, which also means that the same variable can be used for different types:
var x = 6; Var x = ‘Bill’; //x is a string
Simple data types (basic data types)
Numeric base system: common: binary, octal, decimal, hexadecimal
7 var num1 = 07; Var num2 = 019; Var num3 = 08; Hexadecimal number sequence range: 0, 9 and A~F var num = 0xA;
Remember to prefix octal with 0 and hexadecimal with 0x in JS
Numerical range
Maximum and minimum values for numeric values in JavaScript
alert(Number.MAX_VALUE); / / 1.7976931348623157 e+308 alert (Number. MIN_VALUE); //5e-324
Three special values
alert(Infinity); //Infinity, Infinity, greater than any value alert(-infinity); // -infinity, representing infinitesimal, less than any numeric value alert(NaN); //NaN, which represents a non-numeric value
isNaN()
Used to determine whether a variable is of a non-numeric type and returns true or false
var usrAge = 21; var isOk = isNaN(usrAge); console.log(isNum); Var userName = ‘userName ‘; console.log(isNaN(userName)); //true,’ Po ‘is a non-number
String
A string can be any text within quotation marks. The syntax is double quotation marks or single quotation marks
Var strMsg = “I love Beijing tiananmen ~”; Var strMsg2 = ‘WDNMD’; Var strMsg3 = hahaha; // Error without quotation marks is considered js code, but JS does not have this syntax
Because attributes in HTML tags use double quotation marks, we prefer to use single quotation marks
String escape character
A web warning box is displayed
< !DOCTYPE html>
< HTML > < head> < meta charset=” UTF-8 “> < title> < script type=”text/javascript”> alert I look around, here, is my stage, I am the king of heaven and earth. At this moment, I am heroic, and finally shouted out: “rubbish ~”); < /script> < /head> < body> < /body> < /html>
String length
The length property of the string is used to obtain the length of the entire string
Var strMsg = ‘I am Po! ‘; alert(strMsg.length); / / show 5
String splicing
//1.1 string “add” alert(‘hello’ + ‘ ‘+ ‘world’); / / hello world / / 1.2 numerical string “addition” alert (‘ 100 ‘+’ 100 ‘); //100100 //1.3 Value string + value alert(’11’ + 12); / / 1112
+ sign summary formula: add values, join characters
String concatenation enhancement
Console. log(‘ Po ‘+ 18); Var age = 18; //console.log(‘ age is old ‘); Console. log(‘ apo ‘+ age); // console. Log (‘ console ‘+ age +’ age ‘); // Po is 18 years old
The Boolean
Boolean types have two values: true and false, where true means true (true) and false means false (false)
When Boolean and numerals are added, true has the value 1 and false has the value 0
console.log(true + 1); //2 console.log(false + 1); / / 1
Undefined and Null
A declared variable that has not been assigned a value will have a default value of undefined
var variable; console.log(variable); //undefined console.log(‘ hello ‘+ variable); // undefined console.log(11 + variable); //NaN console.log(true + variable); //NaN
A declaration variable is given to a null value, in which the value is null
var vari = null; Console. log(‘ hello ‘+ vari); // hello null console.log(11 + vari); //11 console.log(true + vari); / / 1
Gets the data type Typeof the test variable
Data type conversion
In plain English, a variable of one data type is converted to another data type
There are three conversion modes:
- To a string type
- Convert to an alphanumeric
- Convert to Boolean
Convert to string
Conversion to Alphanumeric (emphasis)
Calculating age cases
Simple adder case
Convert to Boolean
Values representing null and negative are converted to false, such as “, 0, NaN, null, undefined
All other values are converted to true
console.log(Boolean(”)); //false console.log(Boolean(0)); //false console.log(Boolean(NaN)); //false console.log(Boolean(null)); //false console.log(Boolean(undefined)); //false console.log(Boolean(‘ small white ‘)); //true console.log(Boolean(12)); //true
Well, this is the end of today’s article, I hope to help you confused in front of the screen