A variable is a value assigned to an identifier, so you can reference and use it later in the program.

Variables in JavaScript do not have any type attached. They are untyped.

Once you assign a value with some type to a variable, you can later reassign the variable to host a value of any other type, without any issue.

This is because JavaScript is loosely typed.

A variable must be declared before you can use it. There are 3 ways to do this, using var, let or const, and those 3 ways differ in how you can interact with the variable later on.

Using var

Until ES2015, var was the only construct available for defining variables.

var a = 0

If you forget to add var you will be assigning a value to an undeclared variable, and the results might vary.

In modern environments, with strict mode enabled, you will get an error. In older environments (or with strict mode disabled) this will initialize the variable and assign it to the global object.

If you don’t initialize the variable when you declare it, it will have the undefined value until you assign a value to it.

var a //typeof a === 'undefined'

You can redeclare the variable many times, overriding it:

var a = 1
var a = 2

You can also declare multiple variables at once in the same statement:

var a = 1, b = 2

Using let

let is a new feature introduced in ES2015 and it’s essentially a block scoped version of var. Its scope is limited to the block, statement or expression where it’s defined, and all the contained inner blocks. We’ll talk more about scope later.

Modern JavaScript developers might choose to only use let and completely discard the use of var.

If let seems an obscure term, just read let color = 'red' as let the color be red and it all makes much more sense

Using const

Variables declared with var or let can be changed later on in the program, and reassigned. Once a const is initialized, its value can never be changed again, and it can’t be reassigned to a different value.

const a = 'test'

We can’t assign a different literal to the a const. We can however mutate a if it’s an object that provides methods that mutate its contents.

const does not provide immutability, just makes sure that the reference can’t be changed.

const has block scope, same as let.

Modern JavaScript developers might choose to always use const for variables that don’t need to be reassigned later in the program.

Why? Because we should always use the simplest construct available to avoid making errors down the road.

This is an important concept. We should make use of any useful limitation that prevents us to do mistakes down the road.

We’ll later talk about a very important difference between those 3 variable declarations, in regards to scope and hoisting.

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