From 6a725d73849e2e32fe7cfaf67a7a985232945a2a Mon Sep 17 00:00:00 2001
From: Chalarangelo <chalarangelo@gmail.com>
Date: Tue, 7 Sep 2021 12:17:34 +0300
Subject: [PATCH] Add JS question about variable keywords

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 blog_posts/javascript-var-let-const.md | 77 ++++++++++++++++++++++++++
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+---
+title: What are the differences between var, let and const in JavaScript?
+type: story
+tags: javascript,type,variable
+authors: chalarangelo
+cover: blog_images/highlands.jpg
+excerpt: JavaScript variables can be declared a handful of ways. However, they're not all created equal and understanding their differences can drastically change the way you code.
+firstSeen: 2021-09-19T05:00:00-04:00
+---
+
+JavaScript has three variable declaration statements: `var`, `let` and `const`. The latter two were added in ES6, whereas `var` existed since previous versions. One of the first things to notice is that `const` defines constants (i.e. values that will not be reassigned), whereas `var` and `let` define variables. Yet, `var` behaves differently from both `let` and `const` in various other ways.
+
+### Scope
+
+Variables declared with `var` are function scoped, in contrast to variables declared with `let` or `const` which are block scoped.
+
+```js
+const scopeExample = () => {
+  var a = 'var';
+  let b = 'let';
+  console.log(a, b); // 'var', 'let'
+
+  {
+    var c = 'var';
+    let d = 'let';
+    console.log(c, d); // 'var', 'let'
+  }
+
+  console.log(c); // 'var'
+  console.log(d); // Throws a ReferenceError
+};
+```
+
+If you want to learn more, we have [an article covering JavaScript variables and scopes in more depth](/articles/s/javascript-variable-scope).
+
+### Hoisting
+
+While variables declared with `var` are hoisted to the enclosing scope, variables declared with `let` or `const` are not initialized until their definition is evaluated.
+
+```js
+const hoistingExample = () => {
+  console.log(a); // undefined
+  var a = 'var';
+  console.log(a); // 'var'
+
+  console.log(b); // ReferenceError
+  let b = 'let';
+  console.log(b); // 'let'
+};
+```
+
+If you want to learn more, we have [an article covering JavaScript hoisting in more depth](/articles/s/javascript-variable-hoisting).
+
+### Global object property
+
+At the top level, variables declared with `var`, unlike ones declared with `let` or `const`, create a property on the global object.
+
+```js
+var a = 'var';
+let b = 'let';
+
+console.log(window.a); // 'var'
+console.log(window.b); // undefined
+```
+
+### Redeclaration
+
+In strict mode, variables declared with `var` can be re-declared in the same scope, whereas this is not allowed for variables declared with `let` or `const`.
+
+```js
+'use strict';
+var a = 'var1';
+var a = 'var2';
+
+let b = 'let1';
+let b = 'let2'; // SyntaxError
+```