javscript let 是 javascript 1.7 引进的关键字。和 var 类似，区别在于：

• 作用域
• firefox 支持，而目前 chrome(34) 不支持。

function allyIlliterate() {
    //tuce 不可用

    for( let tuce = 0; tuce < 5; tuce++ ) {
        //tuce is only visible in here (and in the for() parentheses)
    };

    //tuce 不可用
};

function byE40() {
    //nish 存在

    for( var nish = 0; nish < 5; nish++ ) {
        //nish is visible to the whole function
    };

    //nish 存在
};

let 也可用于产生 闭环

function conjunctionJunctionWhatsYour() {
    //sNotGetCrazy is *not* visible out here

    let( sNotGetCrazy = 'now' ) {
        //sNotGetCrazy is only visible in here
    };

    //sNotGetCrazy is *not* visible out here
};

参考：

https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Statements/let