js中let和var如何定义变量

let hello = 'hello world.';

console.log(hello);

let hello = 'hello world.';

^^^

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

 ...

'use strict';

let hello = 'hello world.';

console.log(hello);

'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

}());