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】

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