2016-01-05-differences-between-undefined-and.md

title: Differences between undefined and null tip-number: 05 tip-username: loverajoel tip-username-profile: https://github.com/loverajoel tip-tldr: Understanding the differences between undefined and null. tip-writer-support: https://www.coinbase.com/loverajoel

• /en/differences-between-undefined-and-null/

• undefined means a variable has not been declared, or has been declared but has not yet been assigned a value

• null is an assignment value that means "no value"

• Javascript sets unassigned variables with a default value of undefined

• Javascript never sets a value to null. It is used by programmers to indicate that a var has no value.

• undefined is not valid in JSON while null is

• undefined typeof is undefined

• null typeof is an object. Why?

• Both are primitives

• Both are falsy (Boolean(undefined) // false, Boolean(null) // false)

• You can know if a variable is undefined

  typeof variable === 'undefined';

- You can check if a variable is [null](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/null)

  ```javascript
  variable === null

• The equality operator considers them equal, but the identity doesn't

  null == undefined; // true
  
  null === undefined; // false