Type aliases

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

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

You will apply Type aliases 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 Type aliases in your own words.

type ID = string creates an alias. Unlike interfaces, aliases can name unions, tuples, and mapped types.

interface vs type

Use interfaces for object contracts you may extend; use type aliases for unions and utility compositions.

Composition patterns

Aliases shine for unions of interfaces, mapped types, and conditional types—patterns interfaces cannot express. Brand types (type UserId = string & { __brand: 'UserId' }) are an advanced pattern to prevent mixing IDs.

Self-check

Model Result<T> as success | failure with a literal ok field.

Practice: Apply type-aliases in the playground, then explain type aliases 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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