The utility types in TypeScript

The built-ins worth memorising, and what each one is for.

interface Run {
  lang: string;
  tpm: number;
  stars: number;
  note?: string;
}

type Summary = Pick<Run, "lang" | "tpm">;
type WithoutNote = Omit<Run, "note">;
type ByLang = Record<string, Summary>;
type Patch = Partial<Run>;
type Complete = Required<Run>;

async function load(): Promise<Run[]> {
  return [];
}

type Loaded = Awaited<ReturnType<typeof load>>;
type Args = Parameters<(lang: string, tpm: number) => void>;

const summary: Summary = { lang: "ts", tpm: 98 };
const table: ByLang = { ts: summary };
const patch: Patch = { stars: 3 };
const rows: Loaded = [];
const args: Args = ["ts", 98];
console.log(table.ts?.tpm, patch.stars, rows.length, args[0]);

How it works

1. Pick and Omit select or drop keys.
2. Record builds a map type; Awaited unwraps a promise.
3. ReturnType and Parameters read a function's type.

Keywords and builtins used here

• Args
• ByLang
• Loaded
• Patch
• Promise
• Summary
• async
• const
• function
• interface
• number
• return
• string
• summary
• type

The run, in numbers

Lines

26

Characters to type

660

Tokens

196

Three-star pace

100 tpm

At the three-star pace of 100 tokens a minute, this run takes about 118 seconds.

Type this snippet

Step 4 of 4 in Reading types, step 8 of 20 in Generics.

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