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TypeScript in Depth · Interfaces & Types · Lesson 4 of 16

Defining Interfaces

Concept

Defining Interfaces

An interface describes the shape of an object, meaning which properties it must have and their types.

By the end of this lesson
  • Explain the core idea behind Defining Interfaces.
  • Predict output before running a change.
  • Test one realistic and one unusual input.
  • Use the result to make the next decision.
How to study this page
  1. 1. Read one concept.
  2. 2. Change the example.
  3. 3. Run, compare, and explain.
  4. 4. Complete the challenge below.
ExampleRunnable
interface Student {
  name: string;
  age: number;
}
const s: Student = { name: "Alex", age: 20 };
Try it Yourself »
Self-check before continuing

Without looking at the example, describe what changes when you modify one input in Defining Interfaces. Then reopen the editor and prove your explanation with a small test.

You are ready to continue when you can predict, test, and explain the result.

Your turn

Define an interface Course with title, level, and totalLessons fields.

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Console

Output will appear here...