TypeScript Foundations·Types that pay for themselves

Interfaces and type aliases

Describe object shapes once and reuse the contract across your application.

Intermediate 13 min read Lesson 7 of 300 Challenge included

What you will be able to do

Use optional properties for genuinely absent data, not for lazy modelling.
readonly prevents accidental mutation at the type level.
Interfaces can extend other interfaces to compose contracts.

In plain English

Interfaces and type aliases both describe shapes. Interfaces are open to declaration merging and read naturally for objects; aliases can describe unions, tuples and primitives. Pick one convention and stay consistent.

models.tsTypeScript

interface Lesson {
  id: string
  title: string
  minutes: number
  published?: boolean
}

interface Course extends Lesson {
  readonly slug: string
  lessons: Lesson[]
}

const html: Course = {
  id: 'html',
  slug: 'html',
  title: 'HTML Foundations',
  minutes: 120,
  lessons: [],
}

Your turn

Define a credential

Easy

Create a Credential interface with id, holder, program and an optional score. Then create one valid object.

TS · starter

interface Credential {
  // add fields
}

Check your understanding before moving on.

Knowledge checkQuestion 1

Which declaration can describe a union of string literals?

Select an answer to continue

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Next: Unions and narrowing

Lesson 7 of 300 · 0% of course