JavaScript Var、Let 和Const 有什么区别

ES2015（ES6）推出了许多闪亮的新功能。从 2020 年开始，我们假设许多 JavaScript 开发人员已经熟悉并开始使用这些功能。

尽管这个假设可能部分正确，但是其中某些功能可能对一些开发人员来说仍然是个谜。

ES6 附带的功能之一是添加了let和const，可用于变量声明。问题是，它们与var有何不同？ 如果你仍然不清楚-那么读完本文你就知道了 😂。

在本文中，我们将讨论var，let和const的作用域、用途和变量提升。当你阅读时，请注意我将指出的它们之间的差异。

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Javascript let vs var: The Difference and Comparison

Javascript let, and var are both used for variable declaration. Before the ES6, JavaScript had only two types of scope:

1. Global Scope
2. Function Scope

Global Scope

Variables that are declared Globally (outside any function) have Global Scope.

var netflixFilm = "enola holms";

// code here can use netflixFilm

function detective() {
  // code here can also use netflixFilm
}

In this example, the variable is defined outside the function. So it is in the global scope.

You can access global variables anywhere in JavaScript programs.

#javascript #javascript let #javascript let vs var

JavaScript Var, Let, and Const - 8

To declare a variable in JavaScript either var, let or const is used.
We will distinguish between the three with the following features:

• Block scope
• Update and redeclaration
• Hoisting
• Undefined

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#javascript #javascript var #let #const

JavaScript Variables: var and let and const

There are three ways to create variables in a JavaScript application: using var, using let, or using const. This will not be a post trying to convince you which one you should use, or arguing about what is best. It’s just good to know about the differences and what it means when you use the different options. But hopefully by the end of all this you’ll be comfortable with the three options and can make a decision for your team that will suit your needs. To get the most out of this post, it is best if you understand variable scope, which we covered in this post previously.

#javascript #var #let #const

Declaring A Winner Between JavaScript's var, let And const

When ECMAScript 6 (also known as ECMAScript 2015) was released a collection of new APIs, programming patterns and language changes became a standard. Since ES6 started gaining browser and nodejs support developers are wondering if they should stop using the traditional var to declare variables.
ES6 introduced two new ways to declare variables, let and const.

var - has function level scoping and can change the value reference
let - has block level scoping and can change the value reference
const - has block level scoping but cannot change the value reference
Both provide better block scoping that var. const differs from let because the immediate value cannot be changed once it is declared.
Variables declared using var are function scoped, which has led to confusion to many developers as they start using in JavaScript.

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A Simple Explanation Of JavaScript Variables: Const, Let, Var

The variable is a fundamental concept that any developer should know.

In JavaScript, const, let, and var are the statements you can declarate variable.

I’m going to describe each variable type around the declaration, initialization, value access, and assignment. Each of the 3 types (const, let, and var) create variables that behave differently exactly in these 4 steps.

This post isn’t quite beginner friendly, but rather useful to solidify your knowledge of variables and their behavior.

Let’s get started.

1. Variable identifier

First, let’s understand what a variable is.

In simple terms, a variable is a placeholder (or a box) for a value. A value in JavaScript can be either a primitive value or an object.

The variable has a name, which stricter is called identifier. Examples of identifiers are myNumber, name, list, item.

The syntax of an identifier is pretty simple:

An identifier can contain letters, digits 0..9, and special symbols $, _. An identifier cannot start with a digit 0..9.

Examples of valid identifiers are myNumber, my_number, list1, $item, _name, a, b, $, _.

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