Javascript Type

typeof and instanceof are both operators in JavaScript that are used to check the type of a value or an object. However, they work differently and have different use cases.

typeof

Returns a string indicating the type of an operand.

typeof "Hello World" // => "string"
typeof 42 // => "number"
typeof true // => "boolean"
 
typeof undefined // => "undefined"
typeof null // => "object"
 
typeof [1, 2, 3] // => "object"
typeof { name: "John", age: 30 } // => "object"

Note:

  • typeof null returns "object", which is a known quirk of JavaScript. This is because null is considered an empty object reference in JavaScript.

instanceof

Checks if an object is an instance of a particular class.

var today = new Date();
today instanceof Date // => true
today instanceof Object // => true
 
var names = ["John", "Paul", "George", "Ringo"];
names instanceof Array // => true
names instanceof Object // => true
 
var person = { name: "John", age: 30 };
person instanceof Object // => true
person instanceof Array // => false

In the above example,

  • today is an instance of the Date class,
  • names is an instance of the Array class, and
  • person is an instance of the Object class.

keyof

The keyof operator is used in TypeScript to get a union of all the keys of an object type. It returns the names of properties of a given object type.

type Person = {
  name: string;
  age: number;
  isEmployed: boolean;
};
 
// keyof Person would be the union of the property names:
type PersonKeys = keyof Person; // => "name" | "age" | "isEmployed"

Useful when you need to enforce that a variable or argument corresponds to a key in an object, helping improve type safety in TypeScript.

function getValue<T, K extends keyof T>(obj: T, key: K): T[K] {
  return obj[key];
}
 
const person: Person = { name: "John", age: 30, isEmployed: true };
 
// Valid keys
const name = getValue(person, "name");  // => "John"
const age = getValue(person, "age");    // => 30
 
// Invalid key
// const invalid = getValue(person, "invalidKey"); // Error: Argument of type '"invalidKey"' is not assignable to parameter of type '"name" | "age" | "isEmployed"'

Note:

keyof is only available in TypeScript and helps you enforce correct key names when accessing object properties.