标签：错误 重复 orm 覆盖 reference define 使用 run ring
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》
‘use strict‘; (function() { var varTest; let letTest; console.log(varTest); //输出undefined console.log(letTest); //输出undefined }());
(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.‘; }());
‘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); }());
‘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 } }
标签：错误 重复 orm 覆盖 reference define 使用 run ring
原文地址：http://www.cnblogs.com/gamedaybyday/p/6067830.html
