← Back tothis vs that

null vs undefined

Written byPhuoc Nguyen
18 Jun, 2020


  1. `undefined` indicates that a variable has been declared but not been assigned to any value, including `null`.
    let foo;
    console.log(foo); // undefined
    `null` is used to indicate that a variable doesn't have a value.
    let foo = null;
    console.log(foo); // null
  2. `undefined` and `null` are considered to be different types.
    `undefined` is a type itself, whereas `null` is a special value of object.
    console.log(typeof undefined); // 'undefined'
    console.log(typeof null); // 'object'
    Since they are different types, here are the result of equality and identity operators when comparing them to each other:
    null == undefined; // true
    null === undefined; // false

Good to know

`JSON.stringify` omits `undefined`, but keeps `null`:
name: 'John',
address: null,
age: undefined,

// {"name":"John","address":null}
If you found this post helpful, please consider giving the repository a star on GitHub or sharing the post on your favorite social networks 😍. Your support would mean a lot to me!

Questions? 🙋

Do you have any questions about front-end development? If so, feel free to create a new issue on GitHub using the button below. I'm happy to help with any topic you'd like to learn more about, even beyond what's covered in this post.
While I have a long list of upcoming topics, I'm always eager to prioritize your questions and ideas for future content. Let's learn and grow together! Sharing knowledge is the best way to elevate ourselves 🥷.
Ask me questions

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