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TypeScript Keyof


keyof is a keyword in TypeScript which is used to extract the key type from an object type.


keyof with explicit keys

When used on an object type with explicit keys, keyof creates a union type with those keys.

Example

interface Person {
  name: string;
  age: number;
}
// `keyof Person` here creates a union type of "name" and "age", other strings will not be allowed
function printPersonProperty(person: Person, property: keyof Person) {
  console.log(`Printing person property ${property}: "${person[property]}"`);
}
let person = {
  name: "Max",
  age: 27
};
printPersonProperty(person, "name"); // Printing person property name: "Max"
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keyof with index signatures

keyof can also be used with index signatures to extract the index type.

type StringMap = { [key: string]: unknown };
// `keyof StringMap` resolves to `string` here
function createStringPair(property: keyof StringMap, value: string): StringMap {
  return { [property]: value };
}