使用“let”和“var”之间有什么区别？

Question

使用“let”和“var”之间有什么区别？

ECMAScript 6介绍了该let声明。

我听说它被描述为“本地”变量，但我仍然不太确定它与var关键字的行为有何不同。

有什么区别？何时应该let使用var？

Answer 1

let也可用于避免闭包问题。它绑定了新的价值而不是保留旧的参考，如下面的例子所示。DEMOfor(var&nbsp;i&nbsp;=&nbsp;1;&nbsp;i&nbsp;<&nbsp;6;&nbsp;i++)&nbsp;{ &nbsp;&nbsp;document.getElementById('my-element'&nbsp;+&nbsp;i) &nbsp;&nbsp;&nbsp;&nbsp;.addEventListener('click',&nbsp;function()&nbsp;{&nbsp;alert(i)&nbsp;})}上面的代码演示了一个典型的JavaScript闭包问题&nbsp;对i变量的引用存储在单击处理程序闭包中，而不是实际值i。每个单击处理程序都将引用同一个对象，因为只有一个计数器对象可以容纳6，因此每次单击时会得到6个。一般的解决方法是将它包装在匿名函数中并i作为参数传递。现在也可以通过使用let替代var方法来避免这些问题，如下面的代码所示。演示（在Chrome和Firefox 50中测试）'use&nbsp;strict';for(let&nbsp;i&nbsp;=&nbsp;1;&nbsp;i&nbsp;<&nbsp;6;&nbsp;i++)&nbsp;{ &nbsp;&nbsp;document.getElementById('my-element'&nbsp;+&nbsp;i) &nbsp;&nbsp;&nbsp;&nbsp;.addEventListener('click',&nbsp;function()&nbsp;{&nbsp;alert(i)&nbsp;})}

Answer 2

let和之间有什么区别var？从函数的开头，使用var语句定义的变量在其定义的函数中是已知的。（*）使用let语句定义的变量仅在其定义的块中已知，从定义之后开始。（**）要了解其中的差异，请考虑以下代码：// i IS NOT known here// j IS NOT known here// k IS known here, but undefined// l IS NOT known herefunction loop(arr) {&nbsp; &nbsp; // i IS known here, but undefined&nbsp; &nbsp; // j IS NOT known here&nbsp; &nbsp; // k IS known here, but has a value only the second time loop is called&nbsp; &nbsp; // l IS NOT known here&nbsp; &nbsp; for( var i = 0; i < arr.length; i++ ) {&nbsp; &nbsp; &nbsp; &nbsp; // i IS known here, and has a value&nbsp; &nbsp; &nbsp; &nbsp; // j IS NOT known here&nbsp; &nbsp; &nbsp; &nbsp; // k IS known here, but has a value only the second time loop is called&nbsp; &nbsp; &nbsp; &nbsp; // l IS NOT known here&nbsp; &nbsp; };&nbsp; &nbsp; // i IS known here, and has a value&nbsp; &nbsp; // j IS NOT known here&nbsp; &nbsp; // k IS known here, but has a value only the second time loop is called&nbsp; &nbsp; // l IS NOT known here&nbsp; &nbsp; for( let j = 0; j < arr.length; j++ ) {&nbsp; &nbsp; &nbsp; &nbsp; // i IS known here, and has a value&nbsp; &nbsp; &nbsp; &nbsp; // j IS known here, and has a value&nbsp; &nbsp; &nbsp; &nbsp; // k IS known here, but has a value only the second time loop is called&nbsp; &nbsp; &nbsp; &nbsp; // l IS NOT known here&nbsp; &nbsp; };&nbsp; &nbsp; // i IS known here, and has a value&nbsp; &nbsp; // j IS NOT known here&nbsp; &nbsp; // k IS known here, but has a value only the second time loop is called&nbsp; &nbsp; // l IS NOT known here}loop([1,2,3,4]);for( var k = 0; k < arr.length; k++ ) {&nbsp; &nbsp; // i IS NOT known here&nbsp; &nbsp; // j IS NOT known here&nbsp; &nbsp; // k IS known here, and has a value&nbsp; &nbsp; // l IS NOT known here};for( let l = 0; l < arr.length; l++ ) {&nbsp; &nbsp; // i IS NOT known here&nbsp; &nbsp; // j IS NOT known here&nbsp; &nbsp; // k IS known here, and has a value&nbsp; &nbsp; // 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在这里，我们可以看到我们的变量j只在第一个for循环中被知道，而不是在之前和之后。然而，我们的变量i在整个函数中是已知的。另外，请考虑块范围变量在声明之前是未知的，因为它们不会被提升。您也不允许在同一块中重新声明相同的块范围变量。这使得块范围变量比全局或功能范围变量更不容易出错，这些变量被提升并且在多个声明的情况下不会产生任何错误。let今天使用安全吗？有些人会争辩说，将来我们只会使用let语句，而var语句将会过时。JavaScript大师凯尔辛普森写了一篇非常精细的文章，说明为什么他认为情况并非如此。然而，今天绝对不是这样。事实上，我们实际上需要问自己，使用该let陈述是否安全。这个问题的答案取决于您的环境：如果您正在编写服务器端JavaScript代码（Node.js），则可以安全地使用该let语句。如果您正在编写客户端JavaScript代码并使用基于浏览器的转换器（如Traceur或babel-standalone），则可以安全地使用该let语句，但是您的代码在性能方面可能不是最优的。如果您正在编写客户端JavaScript代码并使用基于节点的转换器（如traceur shell脚本或Babel），则可以安全地使用该let语句。而且因为您的浏览器只会知道转换后的代码，所以应该限制性能缺陷。如果您正在编写客户端JavaScript代码而不使用转换器，则需要考虑浏览器支持。仍然有一些浏览器根本不支持let：如何跟踪浏览器支持有关let在阅读本答案时哪些浏览器支持该语句的最新概述，请参阅此Can I Use页面。（*）全局和功能范围的变量可以在声明之前初始化和使用，因为JavaScript变量是悬挂的。这意味着声明始终位于范围的顶部。（**）未提升块范围变量