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let,const用于声明变量,用来替代老语法的var关键字,与var不同的是,let/const会创建一个块级作用域{}
var在全局代码中执行, window 下 const和let只能在代码块中执行, script词法作用域下 const声明常量后必须立马赋值 let声明变量后可立马赋值或使用时赋值 不允许重复声明 未定义就使用会报错:const和let不存在变量提升 暂时性死区: 在代码块内使用const和let声明变量之前,该变量都不可用
var、const、let、function、class、import
// 字符串 const [a, b, c, d, e] = "hello" // 数值, 先转包装对象; v -> f() const { valueOf: v } = 123 // 布尔, 先转包装对象 const { valueOf: v } = false // 对象 const { x, y } = { x: 1, y: 2 } const { x, y = 2 } = { x: 1 } // y 为默认值 const { x, y: z } = { x: 1, y: 2 } // z 为改名变量 // 数组 const [x, y] = [1, 2] const [x, y = 2] = [1] // 默认 y // 函数参数 function fn([x = 0, y = 1] = []) { console.log(x, y) } // 数组参数 function fn({ x = 0, y = 1 } = {}) { console.log(x, y) } // 对象参数
交换变量值: [x, y] = [y, x] 返回函数多个值: const [x, y, z] = fn() 定义函数参数: fn([1, 2]) 提取JSON数据: const { name, version } = packageJson 定义函数参数默认值: function fn({ x = 1, y = 2 } = {}) {} 遍历Map结构: for (let [k, v] of Map) {} 输入模块指定属性和方法: const { readFile, writeFile } = require("fs")
匹配模式: 只要等号两边的模式相同,左边的变量就会被赋予对应的值 解构赋值规则: 只要等号右边的值不是对象或数组,就先将其转为对象 解构默认值生效条件: 属性值严格等于undefined 解构遵循匹配模式 解构不成功时变量的值等于undefined undefined和null无法转为对象,因此无法进行解构
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1.变量声明
let,const用于声明变量,用来替代老语法的var关键字,与var不同的是,let/const会创建一个块级作用域{}
2.解构赋值
The text was updated successfully, but these errors were encountered: