keyof & typeof Operators

TypeScript features two fundamental type query operators: keyof (returns a union of object property keys) and typeof (extracts the static type from a runtime value).

1. The keyof Operator

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

2. The Type-Level typeof Operator

In JavaScript, typeof evaluates runtime type strings ("string", "object"). In TypeScript, typeof used in a type position extracts the static TypeScript type of a runtime variable, function, or object literal.

3. Combining keyof typeof (Extract Keys from Runtime Objects)

Combining keyof and typeof allows you to extract property key unions directly from runtime objects without duplicating type definitions:

4. Interactive Code Playground

Test keyof and typeof operators below:

5. Summary Table

| keyof typeof | Type Context | Runtime Object Variable | Union of runtime object property keys |