js 变量定义方式

Var（万能）

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（局部使用）

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（只能看不能动）

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