注:如果要使用let声明变量,必须在文件头添加'use strict'严格模式的声明,不然会报错.

1.声明后未赋值,表现相同

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'use strict';

(function() {
var varTest;
let letTest;
console.log(varTest); //输出undefined
console.log(letTest); //输出undefined
}());

2.使用未声明的变量,表现不同:

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'use strict';

(function() {
console.log(varTest); //输出undefined(注意要注释掉下面一行才能运行)
console.log(letTest); //直接报错: Uncaught ReferenceError: letTest is not defined

var varTest = 'test var OK.';
let letTest = 'test let OK.';
}());

3.重复声明同一个变量时,表现不同:

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'use strict';

(function() {
var varTest = 'test var OK.';
var varTest = 'varTest changed.';
console.log(varTest); //输出varTest changed.
}());

(function() {
let letTest = 'test let OK.';
let letTest = 'letTest changed.';
console.log(letTest); ///直接报错: Uncaught SyntaxError: Identifier 'letTest' has already been declared
}());

4.变量作用范围,表现不同

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'use strict';

(function() {
var varTest = 'test var OK.';
let letTest = 'test let OK.';

{
var varTest = 'varTest changed.';
let letTest = 'letTest changed.';
}

console.log(varTest); //输出"varTest changed.",内部"{}"中声明的varTest变量覆盖外部的letTest声明
console.log(letTest); //输出"test let OK.",内部"{}"中声明的letTest和外部的letTest不是同一个变量
}());