Nullability

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 Nullability—the ideas, syntax, and habits you need before moving on in TypeScript.

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

You will apply Nullability 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. Also complete the hands-on challenge before advancing.

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

With strictNullChecks, null and undefined are not assignable unless the type allows them. Use optional chaining ?. and nullish coalescing ??.

Defensive patterns

value ?? default only replaces null/undefined, not 0 or ''. Combine with explicit guards when empty string is invalid.

Self-check

Explain difference between || and ?? with an example.

Optional properties (nickname?: string) differ from string | undefined when exact optional property types are enabled in strict config.

Practice: Apply nullability in the playground, then explain nullability in one sentence without looking at notes.

Playground

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

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