let关键字

作用:
   与var类似, 用于声明一个变量
特点:
    只在块作用域内有效
    不能重复声明
    不会预处理, 不存在提升
应用:
   循环遍历加监听

//应用实例

<body>
    <button>测试1</button><br>
    <button>测试2</button><br>
    <button>测试3</button><br>
</body>

console.log(a); //undefined   (变量/函数)提升
    //不会预处理, 不存在提升
//console.log(b); //Uncaught ReferenceError: b is not defined

var a = 2;
let b = 3;
console.log(a, b);

if(true) {
    var c = 4;
    let d = 5;
    var c = 6;
        //不能重复声明
    //let d = 7; //Uncaught SyntaxError: Identifier 'd' has already been declared
    console.log(c, d);
}
console.log(c);
    //只在块作用域内有效
//console.log(d); //Uncaught ReferenceError: d is not defined

//循环遍历加监听
var buttons = document.getElementsByTagName("button");
for (let i = 0,length=buttons.length; i<length; i++) {
    buttons[i].onclick = function () {
        alert('点击'+(i+1)+'个按钮');
    };
}

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