TypeScript Basics›Core· 30 min read

Basic Types: string, number, boolean, array

Label your values with the four everyday types and let TypeScript guard them.

What you will learn

Annotate variables with string, number and boolean
Type arrays two different ways
See what happens when a value breaks its type

The everyday types

You add a type to a variable by writing a colon and the type name after its name: let age: number. These are the four you will use constantly:

Type	Holds	Example value
`string`	Text	'hello', "Asha"
`number`	Any number (int or decimal)	42, 3.14
`boolean`	true or false	true
`array`	A list of one type	[1, 2, 3]

A worked example

Here is a little profile, each variable clearly typed:

Three basic types in action

let username: string = 'asha_codes';
let age: number = 21;
let isStudent: boolean = true;

console.log(username, age, isStudent);

Note: Output: asha_codes 21 true Each variable now has a fixed type. If you later wrote age = 'old', TypeScript would flag it immediately.

Typing arrays

An array holds many values of the same type. There are two ways to write it; both mean exactly the same thing:

Two ways to type an array

// "list of numbers" — two equivalent styles
let scores: number[] = [90, 85, 100];
let names: Array<string> = ['Asha', 'Ben'];

scores.push(95);          // fine — 95 is a number
console.log(scores);

Note: Output: [ 90, 85, 100, 95 ] number[] and Array<number> mean the same thing. Pushing a number is allowed; pushing a string would be an error.

When a value breaks its type

Pushing the wrong type

let scores: number[] = [90, 85, 100];
scores.push('A+');        // mistake: that is a string!

Note: Output: Error: Argument of type 'string' is not assignable to parameter of type 'number'. The array was declared as numbers only, so a string is rejected — the bug is caught before it can spread.

Tip: Read the colon as "is a". age: number means "age is a number". Read number[] as "an array of numbers".

Watch out: Coming from other languages you might expect int and float. TypeScript has just one number type that covers both 42 and 3.14 — the same as JavaScript. Likewise the type names are lowercase: write string, not String (the capitalised String is something different and best avoided).

Q. Which type would you use for a list of usernames like ["asha", "ben"]?

Answer: It is a list (array) of text values, so the type is string[] — equivalently Array<string>.

✍️ Practice

Declare a typed price: number, title: string and inStock: boolean for a product.
Make a tags: string[] array, push a valid tag, then try pushing a number and read the error.

🏠 Homework

Write five typed variables describing a movie (title, year, rating, isWatched, genres list) using all four basic types.