使用Let

允许的关键词将变量声明连接到它所包含的任何区块的范围(通常是 {.. } 对),换句话说,允许随意窃取任何区块的范围为其变量声明。

让变量无法在窗口对象中访问,因为它们无法在全球范围内访问。

function a(){
    { // this is the Max Scope for let variable
        let x = 12;
    }
    console.log(x);
}
a(); // Uncaught ReferenceError: x is not defined

当使用VAR

在 ES5 中, var 和 variables 具有函数中的曲线,这意味着在函数内有效,而不是函数本身之外。

function a(){ // this is the Max Scope for var variable
    { 
        var x = 12;
    }
    console.log(x);
}
a(); // 12

如果你想知道更多,继续阅读下面

最著名的面试问题之一的范围也可能足以准确使用下列;

使用Let

for (let i = 0; i < 10 ; i++) {
    setTimeout(
        function a() {
            console.log(i); //print 0 to 9, that is literally AWW!!!
        }, 
        100 * i);
}

当使用VAR

for (var i = 0; i < 10 ; i++) {
    setTimeout(
        function a() {
            console.log(i); //print 10 times 10
        }, 
        100 * i);
}

这是因为在使用VAR时,对于每个曲线 iteration,变量被分解并共享副本。

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。

答案缺少一点:

{
  let a = 123;
};

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

我想将这些关键词链接到执行背景,因为执行背景在这一切中都很重要。执行背景有两个阶段:创作阶段和执行阶段。

在创建阶段的执行背景, var, let 和 const 将仍然存储其变量在记忆中与未确定的值在该执行背景的变量环境中. 差异在执行阶段. 如果您使用参考一个变量定义为 var 之前它被分配一个值,它将只是不确定的。

function a(){
    b;
    let b;
}
a();
> Uncaught ReferenceError: b is not defined

显然,至少在Visual Studio 2015中,TypeScript 1.5,“var”允许一个区块中的相同变量名称的多个声明,而“Let”不。

这不会导致编译错误:

var x = 1;
var x = 2;

这将是:

let x = 1;
let x = 2;

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:

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