The keyof Type Operator

The keyof operator computes a union of the permitted property names of a type. It is especially useful for a getter that must reject misspelled keys before the program runs. The key is a type-level constraint, while bracket access is ordinary JavaScript.

Reading the code: keyof Product is "sku" | "price". K can only be one of those keys. T[K] preserves the corresponding return type, so the first call is inferred as string and the second as number.

Keep the boundary clear: keyof produces possible keys, not an array you can loop over. Use Object.keys for actual runtime enumeration.

Save the code as keyof-type-operator.ts. The result below was captured after compiling this exact example with strict TypeScript checking and executing the emitted JavaScript in Node.js.

TypeScript code

type Product = { sku: string; price: number };
function get<T, K extends keyof T>(item: T, key: K): T[K] {
  return item[key];
}
const product: Product = { sku: "BK-1", price: 120 };
console.log(get(product, "sku"));
console.log(get(product, "price"));

Verified output

BK-1
120

Build: npx tsc --strict --target es2022 --module commonjs --lib es2022,dom,esnext.disposable keyof-type-operator.ts; run: node keyof-type-operator.js. The exact filename may be changed freely. When a multi-file project exists, prefer its tsconfig.json and build scripts.

keyof Product is "sku" | "price". K can only be one of those keys. T[K] preserves the corresponding return type, so the first call is inferred as string and the second as number.

The output shown above comes from the initialized values in this program. Replacing a type annotation alone does not transform a JavaScript value.

keyof produces possible keys, not an array you can loop over. Use Object.keys for actual runtime enumeration.

Check both the TypeScript diagnostic and the emitted JavaScript behavior. A program can compile correctly and still need runtime validation or error handling.

Try get(product, "stock") and explain the compiler error. Add a stock field and retry.

Before running your change, write down the type you expect TypeScript to infer and the output you expect JavaScript to print. Then check both predictions.

The keyof operator computes a union of the permitted property names of a type.

Try the practice change and re-run the exact program to test your understanding.