← Back tothis vs that

hasOwnProperty vs in

Written byPhuoc Nguyen
08 Jul, 2020
Last updated
17 Mar, 2021
The `in` operator and `hasOwnProperty` function are the common ways to check if an objects contains a particular key.
const person = {
name: 'Foo',

'name' in person; // true
person.hasOwnProperty('name'); // true


  1. For inherited properties, `in` will return `true`. `hasOwnProperty`, as the name implies, will check if a property is owned by itself, and ignores the inherited properties.
    Let's reuse the person object from the previous example. Since it's a JavaScript object which has built-in properties such as `constructor`, `__proto__`, the following check return true:
    'constructor' in person; // true
    '__proto__' in person; // true
    'toString' in person; // true
    While `hasOwnProperty` returns `false` when checking against these properties and methods:
    person.hasOwnProperty('constructor'); // false
    person.hasOwnProperty('__proto__'); // false
    person.hasOwnProperty('toString'); // false
  2. For the `get` and `set` methods of a class, `hasOwnProperty` also returns `false`.
    For example, we have a simple class representing triangles:
    class Triangle {
    get vertices() {
    return 3;

    // Create new instance
    const triangle = new Triangle();
    Despite the fact that `vertices` is the property of `triangle`:
    triangle.vertices; // 3
    'vertices' in triangle; // true
    `hasOwnProperty` still ignores it:
    triangle.hasOwnProperty('vertices'); // false

Good practice

In order to check the existence of a property, use `hasOwnProperty`. Use `in` to check the existence of a method.

See also

Questions? 🙋

Do you have any questions? Not just about this specific post, but about any topic in front-end development that you'd like to learn more about? If so, feel free to send me a message on Twitter or send me an email. You can find them at the bottom of this page.
I have a long list of upcoming posts, but your questions or ideas for the next one will be my top priority. Let's learn together! Sharing knowledge is the best way to grow 🥷.

Recent posts ⚡

Newsletter 🔔

If you're into front-end technologies and you want to see more of the content I'm creating, then you might want to consider subscribing to my newsletter.
By subscribing, you'll be the first to know about new articles, products, and exclusive promotions.
Don't worry, I won't spam you. And if you ever change your mind, you can unsubscribe at any time.
Phước Nguyễn