Everyday types

Goal

Read and write the basic types, and know when to let TypeScript work them out.

The basics

let name: string = 'Read'
let times: number = 3
let done: boolean = false
let days: string[] = ['2026-10-16', '2026-10-17']

Inference does most of the work

You can leave most of those annotations out. TypeScript infers the type from the value:

let name = 'Read' // string
const days = ['2026-10-16'] // string[]

Write types where they help a reader: function parameters, return types of exported functions, and data shapes. Let inference handle the rest.

Literal types

A const can never change, so TypeScript gives it the exact value as its type:

const kind = 'daily' // type: 'daily', not just string

That is the seed of something powerful. A type can be a set of exact values:

type Kind = 'daily' | 'weekly'

Template literal types

Streak describes a day as a pattern, in streak/src/types.ts:

/** A calendar day in UTC, written as YYYY-MM-DD. Days sort correctly as plain strings. */
export type Day = `${number}-${number}-${number}`

'2026-10-17' fits. 'yesterday' does not, and the checker says so. (It cannot know that month 13 does not exist; that is a job for the type guard in chapter 07.)

any and unknown

• any switches the checker off for that value. Avoid it: it spreads silently.
• unknown means "could be anything, so prove what it is before you use it". You will use it for outside data in chapter 07.

Check it works

In a new file streak/src/play.ts, write:

import type { Day } from './types.js'

const ok: Day = '2026-10-17'
const nope: Day = 'yesterday'

Run npx tsc -p streak --noEmit and read the error for nope. Then delete the file.

Exercise

Hover over different variables in streak/src/habits.ts and read the inferred types. Find one that you would not have guessed.