ts和js中let和var定义变量的区别
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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 } }
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