TypeScript: keyof reference

In a nutshell

TypeScript’s keyof operator takes an object type and produces a string or numeric literal union of its keys.

This means you can declare a type like:

type SomeObjectType = {1: string, 2: string}

Then use keyof to create another derived type:

type DerivedType = keyof SomeObjectType

DerivedType will now be equal to the numeric literal union of SomeObjectType‘s keys 1 | 2 (1 and 2 are the keys of SomeObjectType). So if you do something like:

const foo: DerivedType = 3

You’ll get the error:

Type '3' is not assignable to type 'keyof SomeObjectType'.(2322)

Because 3 is not assignable to 1 | 2.

But if you do:

const foo: DerivedType = 1

Then all is good because 1 is assignable to 1 | 2.

Try this example in the TS Playground.

Sources

• Keyof Type Operator – TypeScript docs