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Today, the editor will share with you the relevant knowledge points about how to use ES6's let and const commands. The content is detailed and the logic is clear. I believe most people still know too much about this knowledge, so share this article for your reference. I hope you can get something after reading this article. Let's take a look.

1. Let command

① has no block-level scope in js, the variable scope declared by var is the whole function body, and let can play this role.

{let a = 1; var b = 2;} console.log (b); / / 2console.log (a); / / an is not defind

② and let can play this role. In js, the declaration of variables and functions will be promoted to the top of the current scope. This will lead to problems.

Var a = []; / / the function and variable I are declared first, while the global variable I is assigned to 10for (var I = 0; I < 10; iSuppli +) {a [I] = function () {console.log (I);} console.log (I); / / 10a [6] (); / / 10

The use of let solves this problem.

For (let I = 0; I < 10; iTunes +) {a [I] = function () {console.log (I);};} a [6] (); / / 6

Unlike var, ③ let does not have the phenomenon of variable promotion.

Console.log (a); / / an is not definedlet a = 1

④ let does not allow repeated declaration of the same variable within the same block-level scope

/ / error {let a = 10; var a = 1;} / / error {let a = 10; let a = 1;}

2. Const command

① const is also used to declare variables, but to declare constants. Once declared, the value of the constant cannot be changed.

② is the same as let and cannot repeatedly declare the same variable within the same block-level scope.

The scope of ③ const is the same as that of the let command: it is valid only within the block-level scope in which the declaration is located.

Const PI = 3.1415 console.log (PI); / / 3.1415//PI = 3; / / Assignment to constant variable. (constant cannot be assigned) / / const PI = 3.1 PI' has already been declared / Identifier 'PI' has already been declared is all the contents of the article "how to use ES6's let and const commands". Thank you for reading! I believe you will gain a lot after reading this article. The editor will update different knowledge for you every day. If you want to learn more knowledge, please pay attention to the industry information channel.

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