Interfaces

Last reviewed May 28, 2026 Content v20260528

Track mode

client_typescript

Means

In-browser TS

Reading

~1 min

Level

intermediate

This lesson

This lesson teaches Interfaces—the ideas, syntax, and habits you need before moving on in TypeScript.

Without a solid grasp of Interfaces, you will repeat mistakes in TypeScript exercises and on real pages or scripts.

You will apply Interfaces in contexts like: Modern front-end apps, Node APIs, and any team that standardizes on TS-first tooling.

Write TypeScript, compile in the browser, run the emitted JavaScript, and check understanding with MCQs.

When the previous lesson's MCQs feel easy and you can explain Interfaces in your own words.

interface User { id: string; name: string } describes object shapes. Interfaces can extend others and support optional properties with ?.

Declaration merging

Interfaces with the same name merge—useful for augmenting third-party types. Type aliases cannot merge, which is why libraries often export interfaces.

Self-check

Extend a BaseEntity interface with createdAt: string on a Post type.

Challenge

Product card

Define an interface Product with id, title, price.
Create one object and print title — $price.

Done when: formatted price line appears in terminal.

Interview prep

interface vs type alias?: Interfaces excel at object extension; type aliases can express unions, tuples, and mapped types.

Playground

Runs in your browser in a sandboxed frame. Backend runners appear when this track’s profile allows them.

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