ts和js中let和var定义变量的区别

javascript 严格模式

第一次接触let关键字，有一个要非常非常要注意的概念就是”JavaScript 严格模式”，比如下述的代码运行就会报错：

let hello = ‘hello world.‘;
console.log(hello);

错误信息如下：

let hello = ‘hello world.‘;
^^^

SyntaxError: Block-scoped declarations (let, const, function, class) not yet supported outside strict mode
    ...

解决方法就是，在文件头添加”javascript 严格模式”声明：

‘use strict‘;

let hello = ‘hello world.‘;
console.log(hello);

更多更详细的关于”javascript 严格模式”说明，请参考阮一峰的博客 
《Javascript 严格模式详解》

let和var关键字的异同

声明后未赋值，表现相同

‘use strict‘;

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

使用未声明的变量，表现不同:

(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：ReferenceError: letTest is not defined

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

重复声明同一个变量时，表现不同：

‘use strict‘;

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

  var varTest = ‘varTest changed.‘;
  let letTest = ‘letTest changed.‘; //直接报错：SyntaxError: Identifier ‘letTest‘ has already been declared

  console.log(varTest); //输出varTest changed.(注意要注释掉上面letTest变量的重复声明才能运行)
  console.log(letTest);
}());

变量作用范围，表现不同

‘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
}());

Egret下：

使用未定义的let：

class Test {
    public name:string = "Test";
    public run(){
       console.log(a); //该行报错 Block-scoped variable ‘a‘ used before its declaration.
       console.log(b);
         let a;
         var b;
    }
}

重复定义let：

class Test {
    public name:string = "Test";
    public run(){
         let a;  //该行报错  Cannot redeclare block-scoped variable ‘a‘.
         var b;

         let a;
         var b;  //该行报错  Cannot redeclare block-scoped variable ‘a‘.
    }
}

变量范围：

class Test {
    public name:string = "Test";
    public run(){
         let a = 1;
         var b = 2;
         {
          let a = 2;
          var b = 2;
         }
         console.log(a);  //1
         console.log(b);  //2
    }
}