Primitive types and arrays

In plain English

A primitive type describes one simple value. The type string is text, number is any numeric value, and boolean is either true or false. The types null and undefined describe values that are intentionally empty or missing. An array type such as string[] means every item in the list is a string, and the longer Array form means exactly the same thing. A tuple fixes both the length and the order of a small array, and a readonly array promises the list will never be changed.

Step by step

1. 1Decide what a single value is: text, a number, or true or false.
2. 2Annotate it with string, number or boolean after the variable name.
3. 3For a list of values, add square brackets to the element type.
4. 4Use a tuple when the list has a fixed number of slots in a fixed order.
5. 5Mark an array readonly when nothing is allowed to change it.

let name: string = 'Ada'
let score: number = 42
let passed: boolean = true
let empty: null = null
let missing: undefined = undefined

const names: string[] = ['Ada', 'Grace']
const sameNames: Array<string> = ['Alan', 'Katherine']

const pair: [string, number] = ['Ada', 42]

const fixed: readonly string[] = ['Ada', 'Grace']

Common mistakes

• Writing number when the value is text causes a compiler error because a string is not a number.
• Treating null and undefined as the same thing is wrong; null is an intentional empty value and undefined means a value was never set.
• Using string[] to describe an array that holds numbers will fail, because every item in a string[] must be a string.
• Expecting a tuple to work like a normal array is a mistake; a tuple has a fixed length and order.