Variables (JavaScript)

What you'll learn

How to create a variable, give it a value, change it later, and show it with console.log.

Why it matters

Programs need to remember things: a score, a name, a price. A variable is how JavaScript remembers.

Explanation

A variable is a named box that holds a value. You create one with a keyword, a name, and = to put a value inside.

let makes a variable you can change later.
const makes a variable that cannot be re-assigned.

Use const by default. Switch to let only when the value needs to change.

console.log(...) prints a value so you can see it. It is how you "look inside" while learning.

Example

JavaScript

const name = "Asha";      // a text value, cannot be re-assigned
let score = 10;           // a number we will change

console.log(name);        // prints: Asha
console.log(score);       // prints: 10

score = score + 5;        // change the value in the jar
console.log(score);       // prints: 15

console.log("Hi " + name + ", your score is " + score);

Step-by-step walkthrough

const name = "Asha" creates a jar called name holding the text Asha.
let score = 10 creates a jar called score holding 10.
The first two console.log calls print Asha and 10.
score = score + 5 reads the current value (10), adds 5, and puts 15 back in the jar. There is no let the second time because the variable already exists.
The last line joins text and values with + and prints Hi Asha, your score is 15.

Try it yourself

Create a let variable called lives set to 3. Subtract 1 from it and print it.

VariablesJavaScript

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Common mistakes

Changing a const. const x = 1; x = 2; throws a TypeError. Use let if it must change.
Using a variable before creating it, which throws a ReferenceError.
Forgetting quotes around text. const name = Asha looks for a variable called Asha.
Typos in names. score and Score are different variables.

Quick quiz

Question 1 of 3

Which keyword creates a variable you can change later?

constletprint

Question 2 of 3

What does console.log(5 + 5) print?

55105 + 5

Question 3 of 3

What happens if you re-assign a const?

It changes silentlyAn error is thrownIt becomes a let

Summary

Variables store values: let can change, const cannot.
Use const by default.
console.log prints a value so you can see it.

Next: the kinds of values a variable can hold.

Explanation

A variable is a named box that holds a value. You create one with a keyword, a name, and = to put a value inside.

let makes a variable you can change later.

const makes a variable that cannot be re-assigned.

Use const by default. Switch to let only when the value needs to change.

console.log(...) prints a value so you can see it. It is how you "look inside" while learning.

Example

JavaScript

const name = "Asha";      // a text value, cannot be re-assigned
let score = 10;           // a number we will change

console.log(name);        // prints: Asha
console.log(score);       // prints: 10

score = score + 5;        // change the value in the jar
console.log(score);       // prints: 15

console.log("Hi " + name + ", your score is " + score);

Step-by-step walkthrough

const name = "Asha" creates a jar called name holding the text Asha.

let score = 10 creates a jar called score holding 10.

The first two console.log calls print Asha and 10.

score = score + 5 reads the current value (10), adds 5, and puts 15 back in the jar. There is no let the second time because the variable already exists.

The last line joins text and values with + and prints Hi Asha, your score is 15.

Try it yourself

Create a let variable called lives set to 3. Subtract 1 from it and print it.

VariablesJavaScript

Ctrl+Enter to run