How to define variables in JavaScript

This article will give you a detailed introduction to the method of defining variables in JavaScript. It has certain reference value. Friends in need can refer to it. I hope it will be helpful to everyone.

Identifier naming rules

1. Identifiers consist of letters, numbers, ## title underscores, $, Variable names cannot start with a number
2. Identifiers cannot use the built-in keywords if, for, break, var, let, etc.
3. Identifiers should use camel case naming form-words composed of variable names Capitalize the first letter

   var messageAsBoolean = "HHO";

Define variables

How to define variables

    var message   = "qqqq";		
    message       = "qqqq"   
	let message   = "qqqq";
	const message = "aaaaa";

var Defining variables will automatically be promoted to the top scope, let The defined variables will not be promoted to the first line

	function a()
	{
		console.log(a);   // 不会报错，a作用域会自动提升至顶行，内容为	undefined
		var a = "aaaaaa";
	}

Equivalent to:

	function a()
	{
		var a;
		console.log(a);
	 	a = "aaaaaa";
	}

The scope of variables declared by let and var

The scope of variables declared by let is block scope Domain

	function fun()
	{								//函数块作用域的开始
		if(true){ 					//if语句块作用域的开始
		
		} 							//if语句块作用域的结束

	}								//函数块作用域的结束

The variable scope declared by ˆvar is the function scope

	function fun(){ 				//函数作用域的开始

	} 								//函数作用域的结束

Global declaration method

	/* 1. 不使用 let、var等修饰，直接定义的变量 */
		message = "aaaaaaaa";
	/* 2. 在全局变量区申请的变量 */
		var message = "aaaaaa";
		let message = "aaaaa";
	/* 注: js 文件的开始部分，不在任何函数内 */

Conditional declaration

The variable defined by ˆvar will increase the value At the top of the function, duplicate definitions will be replaced*/

	function fun(){
		var name = "lili";
		if(true){
			var name = "hho";            //不会报错，代码类似name = 	"hho";
			console.log(name);           //打印结果为hho
		}
		console.log(name);               //打印结果为hho
	}

Let-defined variables will not be promoted to the top of the function

	function fun(){
		let name = "lili";
		if(true){
			let name = "hho";			//新定义变量name
			console.log(name);			// 打印结果为hho
		}
		console.log(name);				// 打印结果为lili
	}

const-defined variables

Defined variables The attributes are similar to let, but the variable value defined by const cannot be modified

  const name = "hho";					//只可使用，不可修改name变量的值

The scope of the defined variable is the block scope, the same as let

The variable defined by const must be initialized

  const name；    						// 报错

【Recommended learning: javascript advanced tutorial】

The above is the detailed content of How to define variables in JavaScript. For more information, please follow other related articles on the PHP Chinese website!