ECMAScript 6 引入了许可声明。

我听说它被描述为一个当地变量,但我仍然不确定它是如何行为不同于 var 关键词。

什么是差异?什么时候应该被允许使用而不是 var?


当前回答

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;

其他回答

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


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
}
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;

ECMAScript 6 添加了另一个关键字来宣布变量,而不是“放下”。

“Let”和“Let”在“var”上引入的主要目标是,而不是传统的词汇,区块点点点点点点点点点点点点点点点点点点点点点点点点。

主要差异是范围差异,而让它只能在所宣布的范围内可用,如在圆圈中,可在圆圈之外可访问,例如从MDN的文档(MDN的例子):

function varTest() {
  var x = 1;
  if (true) {
    var x = 2;  // same variable!
    console.log(x);  // 2
  }
  console.log(x);  // 2
}

function letTest() {
  let x = 1;
  if (true) {
    let x = 2;  // different variable
    console.log(x);  // 2
  }
  console.log(x);  // 1
}`

var x = 'global';
let y = 'global';
console.log(this.x); // "global"
console.log(this.y); // undefined

当在区块内使用时,请将变量的范围限制在该区块上。

var a = 1;
var b = 2;

if (a === 1) {
  var a = 11; // the scope is global
  let b = 22; // the scope is inside the if-block

  console.log(a);  // 11
  console.log(b);  // 22
} 

console.log(a); // 11
console.log(b); // 2