JS 定义变量方式

var

• 变量可以没有初始值 会报undefined但不会报错
• 变量可以修改
• 变量可以覆盖 (重复定义)
• 函数内重复定义对函数外无影响
• 函数内重新赋值对函数外有影响

    var a = 1;
    var b;
    var c = 2;
    console.log(a);//1
    console.log(b);//undefined
    console.log(c);//2
    var c = 3 ;
    console.log(c);//3
    c = 4;
    console.log(c);//4
    k();
    function k(){
        var c = 5;
        console.log(c);//5
        c = 6;
        console.log(c);//6
    }
    console.log(c);//4
    m();
    function m(){
        c = 7;
    }
    console.log(c);//7

let

• 块级作用域
• 函数内外不影响 避免了全局变量的污染
• 没有得到广泛的支持 (Sublime Text 3 Nodejs 下 Run)

    let a = 1;
    let b;
    let c = 2;
    console.log(a);//1
    console.log(b);//undefined
    console.log(c);//2
    let c = 3 ;//SyntaxError: Identifier 'c' has already been declared
    c = 4;
    console.log(c);//4
    k();
    function k(){
        var c = 5;
        console.log(c);//5
        c = 6;
        console.log(c);//6
    }
    console.log(c);//4

const

• 变量必须有初始值
• 变量值不可更改
• 变量值不可覆盖

    const a = 1;
    const b;//SyntaxError: missing = in const declaration
    const c = 2;
    console.log(a);//1
    console.log(c);//2
    const c = 3 ;//SyntaxError: redeclaration of const c
    c = 4;//TypeError: invalid assignment to const `c'
    k();
    function k(){
        const c = 5;
        console.log(c);//5
    }
    console.log(c);//2

20170506补充

console.log(f);//ReferenceError: f is not defined

没有定义变量直接使用会报错
ReferenceError: f is not defined

console.log(f);//undefined
var f;

变量在使用之后才被定义 对于var来说 会发生变量提升
不会报错而是报没有定义
undefined

console.log(f);//ReferenceError: can't access lexical declaration `f' before initialization
let f;

而对于let的情况 回直接报错
ReferenceError: can't access lexical declaration `f' before initialization