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Typescript keyof

The keyof operator is a TypeScript type operator that takes an object type and returns a  string or numeric literal union type of its keys. In simpler terms, it allows you to get a list of the keys of a given object type as a type itself.

Syntax:

type User = {
  id: number;
  name: string;
  email: string;
};

type UserKeys = keyof User; // 'id' | 'name' | 'email'

Example 1: This example illustrates the basic usage of the keyof operator.

JavaScript 
type Person = {
    name: string;
    age: number;
    address: string;
};

type PersonKeys = keyof Person; // "name" | "age" | "address"

function getProperty<T, K extends keyof T>(obj: T, key: K): T[K] {
    return obj[key];
}

const person: Person = {
    name: 'GFG',
    age: 30,
    address: '123 Main St',
};

// Valid usage
const name = getProperty(person, 'name');
const age = getProperty(person, 'age');

console.log(name);
console.log(age);

Output:

GFG
30

Example 2: keyof can also be used in more advanced scenarios, such as when working with mapped types. Mapped types allow you to create new types by transforming existing ones.

JavaScript 
type Person = {
    name: string;
    age: number;
    address: string;
};

type ReadonlyPerson = {
    readonly [K in keyof Person]: Person[K];
};

const readonlyPerson: ReadonlyPerson = {
    name: 'GFG',
    age: 25,
    address: '456 Another St',
};

// Valid usage
console.log(readonlyPerson.name);

Output:

GFG

 


Reffered: https://www.geeksforgeeks.org

TypeScript

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Category:
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Sub Category:
 Tutorial
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