Reading types off values in TypeScript

typeof lifts a value's type; ReturnType and InstanceType go further.

const defaults = {
  theme: "default",
  seconds: 60,
  langs: ["ts", "go"],
};

type Defaults = typeof defaults;
type LangList = Defaults["langs"];

class Session {
  constructor(public lang: string) {}
  best(): number {
    return 104;
  }
}

type SessionType = InstanceType<typeof Session>;
type BestReturn = ReturnType<Session["best"]>;

function configure(overrides: Partial<Defaults>): Defaults {
  return { ...defaults, ...overrides };
}

const session: SessionType = new Session("ts");
const best: BestReturn = session.best();
const langs: LangList = ["ts"];
console.log(configure({ seconds: 300 }).seconds, best, langs.length);

How it works

1. typeof value in a type position is the value's type.
2. InstanceType<typeof Class> is what new produces.
3. This keeps one source of truth: the value.

Keywords and builtins used here

• BestReturn
• LangList
• Partial
• SessionType
• class
• const
• constructor
• function
• number
• public
• return
• string
• type

The run, in numbers

Lines

27

Characters to type

619

Tokens

155

Three-star pace

100 tpm

At the three-star pace of 100 tokens a minute, this run takes about 93 seconds.

Type this snippet

Step 2 of 4 in Reading types, step 6 of 20 in Generics.

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