A variable is a value assigned to an identifier, so you can reference and use it later in the program.
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.
A variable must be declared before you can use it. There are 3 ways to do this, using
const, and those 3 ways differ in how you can interact with the variable later on.
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
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.
let and completely discard the use of
letseems an obscure term, just read
let color = 'red'as let the color be red and it all makes much more sense
Variables declared with
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
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.