Variables

Using let

let allows you to declare variables that can be changed later.

js

let bookTitle = 'Persepolis'
console.log(bookTitle)

output

Persepolis

The order of the lines matters.

js

console.log(bookTitle)
let bookTitle = 'Persepolis'

output

ReferenceError: Cannot access 'bookTitle' before initialization

Redefining variables

With let, we can redefine the variable.

js

let bookTitle = 'Persepolis'
console.log(bookTitle)

bookTitle = 'The Kite Runner'
console.log(bookTitle)

output

Persepolis
The Kite Runner

Using const

We can define a variable with const just as with let.

js

const libraryName = 'Central Library'
console.log(libraryName)

output

Central Library

However, because we used const (which is short for "constant"), we cannot change its value.

js

const libraryName = 'Central Library'
console.log(libraryName)

libraryName = 'Quantum Codex' // throws an error

output

TypeError: Assignment to constant variable.

Referencing between variables

A variable which points at another variable does not update its value.

js

let user1 = 'BookishBen99'
let user2 = user1

console.log('User 1:', user1)
console.log('User 2:', user2)

user1 = 'pageturner_mia'

// notice that user2 does not change
console.log('User 1:', user1)
console.log('User 2:', user2)

output

User 1: BookishBen99
User 2: BookishBen99

User 1: pageturner_mia
User 2: BookishBen99