What are the differences between let and var defined variables in js?-JS Tutorial-php.cn

What are the differences between let and var defined variables in js?

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Release： 2018-06-06 17:50:40

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This article mainly introduces the difference between let and var defined variables in js. It is very good and has reference value. Friends in need can refer to it

javascript strict mode

This is the first time I come across the let keyword. One concept that needs to be very, very important to pay attention to is "javascript strict mode". For example, if the following code is run, an error will be reported:

let hello = &#39;hello world.&#39;;
console.log(hello);

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The error message is as follows:

let hello = &#39;hello world.&#39;;
^^^
SyntaxError: Block-scoped declarations (let, const, function, class) not yet supported outside strict mode
 ...

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The solution is to add the "javascript strict mode" statement in the file header:

'use strict';
let hello = &#39;hello world.&#39;;
console.log(hello);

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Similarities and differences between let and var keywords

No value assigned after declaration, the behavior is the same

&#39;use strict&#39;;
(function() {
 var varTest;
 let letTest;
 console.log(varTest); //输出undefined
 console.log(letTest); //输出undefined
}());

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Using undeclared variables, the behavior is different:

(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 = &#39;test var OK.&#39;;
 let letTest = &#39;test let OK.&#39;;
}());

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When the same variable is declared repeatedly, the behavior is different:

&#39;use strict&#39;;
(function() {
 var varTest = &#39;test var OK.&#39;;
 let letTest = &#39;test let OK.&#39;;
 var varTest = &#39;varTest changed.&#39;;
 let letTest = &#39;letTest changed.&#39;; //直接报错：SyntaxError: Identifier &#39;letTest&#39; 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);
}());

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Variable scope, different performances

&#39;use strict&#39;;
(function() {
 var varTest = &#39;test var OK.&#39;;
 let letTest = &#39;test let OK.&#39;;
 {
 var varTest = &#39;varTest changed.&#39;;
 let letTest = &#39;letTest changed.&#39;;
 }
 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
}());

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The above is what I compiled for everyone. I hope it will be helpful to everyone in the future.

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