Yashotantra Academy logo
← TypeScript theorytheory 0/50 · 0%
level 1
Curious · 0 XP
streak
0d
perfect
0
badges
0

100 XP to Apprentice

Types · easy

4. Object types & interfaces

Describing the shape of objects.

1 min read3 questionsup to 50 XPtopic 4/50

Read in another language

AI translates the explanation and quiz text while preserving code and answer scoring.

An `interface` (or an inline object type) describes the properties an object must have, including their types.

ts
interface Wallet {
  address: string;
  balance: number;
  label?: string; // optional
}

function show(w: Wallet): string {
  return `${w.address}: ${w.balance}`;
}

Properties marked with `?` are optional. `readonly` prevents reassignment after creation. Structural typing means any object with the right shape satisfies the interface — there's no need to explicitly "implement" it.

ts
interface Point { readonly x: number; readonly y: number; }
const p: Point = { x: 1, y: 2 };
p.x = 5; // error: readonly

How to study this topic

  1. 1. Read the explanation once end to end, skipping the code.
  2. 2. Re-read the code snippets and say out loud what each line does.
  3. 3. Answer the quiz without scrolling back — that is the real check.
  4. 4. Read every explanation, even for the answers you got right.

Scoring

  • · Up to 30 XP from quiz accuracy.
  • · +15 XP for a perfect first attempt.
  • · +5 XP while a 3-day streak is running.
  • · 75% or higher marks the topic complete.

Check your understanding

  1. 1. What does a `?` after a property name mean in an interface?

  2. 2. TypeScript uses which kind of typing for objects?

  3. 3. What does `readonly` do to a property?

Badges

First light
Pass your first topic
Sharp shooter
3 perfect first-try quizzes
On a roll
Study 3 days in a row
Week warrior
Study 7 days in a row
Grinder
Earn 500 XP
Deep diver
Earn 1500 XP