Utility types — Partial, Pick, Omit, Record, ReturnType, Awaited

Level: 201 · for anyone who has written an object type

Stub — an outline, not a lesson. There is no runnable example behind this page yet, so nothing on it has been through the check that backs every other claim in this library. The questions below are what the finished page has to answer.

One line: Pick<User, "nmae"> is an error, but Omit<User, "nmae"> is accepted and removes nothing: Omit takes any string as a key, so a misspelt key passes silently and surfaces later as a missing-property error.

Keywords: Partial, Pick, Omit, Record, ReturnType, Awaited

What the finished page will answer

• Why is Pick<User, "nmae"> an error while Omit<User, "nmae"> is accepted, and what does the second leave in the type?
• Is Partial<User> deep — may { profile: {} } be a Partial<User> when profile has required fields?
• What is ReturnType<typeof load> for an async function load, and what does wrapping it in Awaited add?
• What does Record<"a" | "b", number> require that Record<string, number> does not?
• How are these types written in the standard library's lib.es5.d.ts, and which are mapped types and which conditional?

Examples it will need

• utility_types_omit_typo_tserror.ts — the TS2344 error for Pick with a misspelt key, and the missing-property error that is the only sign Omit ignored it
• utility_types_partial_is_shallow_tserror.ts — the error for a partial User whose nested object leaves out a required field
• utility_types_awaited_ts.ts — the value an async loader resolves to, stored in a variable typed with Awaited and ReturnType

See also

• Mapped types — how Partial, Pick and Record are built
• Conditional types — how ReturnType and Awaited are built
• async and await — the promises Awaited unwraps
• Object types — the optional and readonly properties these types add and remove

Sources to start from

• TypeScript Handbook — Utility types ↗