以下两个功能可以显示差异:

function varTest() {
    var x = 31;
    if (true) {
        var x = 71;  // Same variable!
        console.log(x);  // 71
    }
    console.log(x);  // 71
}

function letTest() {
    let x = 31;
    if (true) {
        let x = 71;  // Different variable
        console.log(x);  // 71
    }
    console.log(x);  // 31
}

函数 VS 区块范围:

与 var 和 let 的主要区别是,与 var 宣言的变量是函数分解的,而与 let 宣言的函数是区块分解的。

function testVar () {
  if(true) {
    var foo = 'foo';
  }

  console.log(foo);
}

testVar();  
// logs 'foo'


function testLet () {
  if(true) {
    let bar = 'bar';
  }

  console.log(bar);
}

testLet(); 
// reference error
// bar is scoped to the block of the if statement 

当第一个函数测试Var 被称为变量 foo 时,与 var 声明,仍然在 if 声明之外可用。

与Let的变量:

当第二个函数测试Let被称为变量栏,用Let表示,仅可在 if 声明中访问,因为用Let表示的变量是区块分开的(其中一个区块是曲线条之间的代码,例如,如果{},为{},函数{})。

请不要让变量变量:

变量与不要被抓住:

console.log(letVar);

let letVar = 10;
// referenceError, the variable doesn't get hoisted

与 var do 相容的变量:

console.log(varVar);

var varVar = 10;
// logs undefined, the variable gets hoisted

var bar = 5;
let foo  = 10;

console.log(bar); // logs 5
console.log(foo); // logs 10

console.log(window.bar);  
// logs 5, variable added to window object

console.log(window.foo);
// logs undefined, variable not added to window object

使用 var 声明定义的变量在其定义的整个函数中已知,从函数的开始。 (*) 使用 let 声明定义的变量仅在其定义的区块中已知,从其定义的时刻开始。

// i IS NOT known here
// j IS NOT known here
// k IS known here, but undefined
// l IS NOT known here

function loop(arr) {
    // i IS known here, but undefined
    // j IS NOT known here
    // k IS known here, but has a value only the second time loop is called
    // l IS NOT known here

    for( var i = 0; i < arr.length; i++ ) {
        // i IS known here, and has a value
        // j IS NOT known here
        // k IS known here, but has a value only the second time loop is called
        // l IS NOT known here
    };

    // i IS known here, and has a value
    // j IS NOT known here
    // k IS known here, but has a value only the second time loop is called
    // l IS NOT known here

    for( let j = 0; j < arr.length; j++ ) {
        // i IS known here, and has a value
        // j IS known here, and has a value
        // k IS known here, but has a value only the second time loop is called
        // l IS NOT known here
    };

    // i IS known here, and has a value
    // j IS NOT known here
    // k IS known here, but has a value only the second time loop is called
    // l IS NOT known here
}

loop([1,2,3,4]);

for( var k = 0; k < arr.length; k++ ) {
    // i IS NOT known here
    // j IS NOT known here
    // k IS known here, and has a value
    // l IS NOT known here
};

for( let l = 0; l < arr.length; l++ ) {
    // i IS NOT known here
    // j IS NOT known here
    // k IS known here, and has a value
    // l IS known here, and has a value
};

loop([1,2,3,4]);

// i IS NOT known here
// j IS NOT known here
// k IS known here, and has a value
// l IS NOT known here

今天使用安全吗?

有些人会说,在未来,我们只会使用让陈述,而这些陈述会变得过时。JavaScript老师Kyle Simpson写了一篇非常复杂的文章,他认为为什么不会这样。

事实上,我们实际上需要问自己是否安全使用放弃声明,这个问题的答案取决于你的环境:

此分類上一篇

如何跟踪浏览器支持

var   --> Function scope  
let   --> Block scope
const --> Block scope

是的

在此样本中,你可以看到我被宣布在一个如果区块. 但它被宣布使用 var. 因此,它获得功能范围. 它意味着你仍然可以访问变量 i 内部函数 x. 因为 var 总是被推到函数. 尽管变量 i 被宣布在一个如果区块, 因为它使用 var 它被推到主函数 x。

函数 x(){ if(true){ var i = 100; } console.log(i); } x();

现在变量 i 被宣布在函数 y. 因此, i 被定义为函数 y. 您可以访问 i 内部函数 y. 但不是从外部函数 y。

let, const

让和 const 有区块范围。

但是,差异是,当你将值分配给 const 时,你不能再分配。

console.log(x); var x = 100;

console.log(x); // ERROR let x = 100;

var 是全球范围(可容量)的变量。

Let and const 是区块范围。

测试JS

{ let l = 'let'; const c = 'const'; var v = 'var'; v2 = 'var 2'; } console.log(v, this.v); console.log(v2, this.v2); console.log(l); // ReferenceError: l 未定义 console.log(c); // ReferenceError: c 未定义

以下两个功能可以显示差异:

function varTest() {
    var x = 31;
    if (true) {
        var x = 71;  // Same variable!
        console.log(x);  // 71
    }
    console.log(x);  // 71
}

function letTest() {
    let x = 31;
    if (true) {
        let x = 71;  // Different variable
        console.log(x);  // 71
    }
    console.log(x);  // 31
}