为什么要学习ES6
每一次规范的诞生意味着语言的欠缺，性能的增强，javascript自身也有一些令人不称心的中央
- 变量晋升个性减少了程序运行时的不可预测性
- 语法过于涣散，实现雷同的性能，不同的人可能会写出不同的代码
let关键字
- let申明的关键字具备块级作用域，所谓的块级作用域便是一个{}
<!DOCTYPE html><html lang="en"><head> <meta charset="UTF-8"> <title>let申明的变量具备块级作用域</title></head><body></body><script> if (true) { let a = 20; console.log(a); //20 if (true) { let b = 50; console.log(b)//50 } } console.log(a) //a is not define</script></html>- let能够避免循环变量变成全局变量
<!DOCTYPE html><html lang="en"><head> <meta charset="UTF-8"> <title>let能够避免循环变量变成全局变量</title></head><body></body><script> for(let i =0;i<2;i++){ } console.log(i) //i is not define</script></html>- let申明的变量不会进行变量晋升
<!DOCTYPE html><html lang="en"><head> <meta charset="UTF-8"> <title>let申明的变量不会进行变量晋升</title></head><body></body><script> console.log(a) // a is not define let a = 20;</script></html>- let申明的变量具备暂存性死区的个性
<!DOCTYPE html><html lang="en"><head> <meta charset="UTF-8"> <title>let申明的变量具备暂存性死区的个性</title></head><body></body><script> let a = 20; if(true) { console.log(a) let a = 30; }</script></html>let面试之var
此题的关键点：变量i是全局的 函数输入的值是全局作用域下的i<!doctype html><html lang="en"><head> <meta charset="UTF-8"> <meta name="viewport" content="width=device-width, user-scalable=no, initial-scale=1.0, maximum-scale=1.0, minimum-scale=1.0"> <meta http-equiv="X-UA-Compatible" content="ie=edge"> <title>let面试题</title></head><body></body><script> var arr = []; for(var i = 0;i<2;i++){ arr[i] = function () { console.log(i) } } arr[0]()//2 arr[1]()//2</script></html>let面试题之let\
关键点：函数执行的时候，输入的是上一级循环产生的块状作用域下i的值
<!DOCTYPE html><html lang="en"><head> <meta charset="UTF-8"> <title>let面试题</title></head><body></body><script> let arr = []; for(let i =0;i<2;i++){ arr[i]= function () { console.log(i) } } arr[0]()//0 arr[1]()//1</script></html>const
const用来申明常量 常量就是值 （内存地址）不能变动的量
- 应用const申明的常量具备块级作用域的个性
<!DOCTYPE html><html lang="en"><head> <meta charset="UTF-8"> <title>const申明的常量具备块级作用域的个性</title></head><body></body><script> if(true){ const a = 10; if(true) { const a =20; console.log(a)//20 } console.log(a)//10 } console.log(a)// a is not define</script></html>- 应用const申明的常量必须初始值
<!DOCTYPE html><html lang="en"><head> <meta charset="UTF-8"> <title>const申明的常量必须赋初始值</title></head><body></body><script> const a; console.log(a)</script></html>应用const申明的常量值（内存地址）不能更改
<!DOCTYPE html><html lang="en"><head> <meta charset="UTF-8"> <title>const申明的常量值不能更改</title></head><body></body><script> const arr = [200,300]; arr[0]='hello'; arr[1]='word'; console.log(arr);//['hello','word'] arr = [200,500]; console.log(arr) //TypeError: invalid assignment to const `arr'</script></html>