Basic Types

Introduction

TypeScript's type system starts with the same kinds of values JavaScript has. In this lesson you meet them, and the two special types you should use carefully.

The primitive types

The three you will use most are lower case: string, number and boolean.

Always write them in lower case. String with a capital letter is a different, rarely wanted type.

Arrays

An array type says what is inside. Write the element type followed by []:

You may also see Array<number>, which means the same thing.

Tuples

A tuple is an array with a fixed length and a known type at each position.

Tuples are handy when a function returns two related values.

null and undefined

undefined means "no value has been set" and null means "deliberately empty". With TypeScript's strict mode, which these lessons use, you must say when a value might be missing, by using a union with |:

Why does this matter? In JavaScript, calling a method on a missing value crashes with "Cannot read properties of undefined". TypeScript catches this at compile time:

To fix it, check first. Inside the if, TypeScript knows text is a string:

This is called narrowing, and you will see more of it in lesson 5.

any and unknown

Sometimes you really do not know the type, for example when reading JSON from a server.

• any turns type checking off for that value. TypeScript will let you do anything with it, so mistakes sneak back in.
• unknown says "I don't know yet". TypeScript makes you check the value before you use it.

Compare:

Prefer unknown over any. Treat any as a last resort.

Try it yourself

The code below has two type errors. Fix both so that it prints 6.

Summary

• Use lower case string, number and boolean.
• number[] is an array of numbers. A tuple such as [string, number] fixes length and order.
• Say when a value can be missing with string | undefined, then check before using it.
• Prefer unknown to any, because unknown forces a check.