Keyof

the keyof keyword is utilized to extract the type of keys from an object type. When applied to an object type that has explicit keys, keyof generates a union type encompassing those keys.

interface Employee {
  name: string;
  age: number;
}

function printEmployeeProperty(employee: Employee, property: keyof Employee) {
  console.log(
    `Printing employee property ${property}: "${employee[property]}"`
  );
}
// `keyof Employee` here creates a union type of "name" and "age",
// other strings will not be allowed
let employee = {
  name: 'Matt',
  age: 30,
};
printEmployeeProperty(employee, 'name'); // Printing employee
// property name: "Matt"

keyof using index signatures

the keyof keyword can also be employed with index signatures to extract the type of indexes.

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

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