The difference between let and Var defined variables in JS

JavaScript Strict mode

The first contact with the Let keyword, there is a very very important concept is "JavaScript strict mode", such as the following code to run the error:

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

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The error message is as follows:

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

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The workaround is to add a "JavaScript strict mode" declaration to the file header:

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

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For more detailed instructions on "JavaScript strict mode", please refer to Nanyi's blog
A detailed description of Javascript in strict mode

Let and VAR keywords are not assigned after the declaration of similarities and differences, the same performance

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

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Use undeclared variables to behave differently:

(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.‘;}());

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When declaring the same variable repeatedly, it behaves differently:

‘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);}());

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Variable action range, different performance

‘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}());

The difference between let and Var defined variables in JS