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"keyof with index signatures
keyof can also be used with index signatures to extract the index type.
Example
type StringMap = { [key: string]: unknown };
// `keyof StringMap` resolves to `string` here function createStringPair(property: keyof StringMap, value: string): StringMap {
return { [property]: value };
}