What reason is there to use null instead of undefined in JavaScript?

Question

I\'ve been writing JavaScript for quite a long time now, and I have never had a reason to use null. It seems that undefined is always preferable an

Answer 1

Just wanna add that with usage of certain javascript libraries, null and undefined can have unintended consequences.

For example, lodash's get function, which accepts a default value as a 3rd argument:

const user = {
  address: {
    block: null,
    unit: undefined,
  }
}
console.log(_.get(user, 'address.block', 'Default Value')) // prints null
console.log(_.get(user, 'address.unit', 'Default Value')) // prints 'Default Value'
console.log(_.get(user, 'address.postalCode', 'Default Value')) // prints 'Default Value'

Another example: If you use defaultProps in React, if a property is passed null, default props are not used because null is interpreted as a defined value. e.g.

class MyComponent extends React.Component {
   static defaultProps = {
      callback: () => {console.log('COMPONENT MOUNTED')},
   }
   componentDidMount() {
      this.props.callback();
   }
}
//in some other component
   // Console WILL print "COMPONENT MOUNTED"
   // Console will NOT print "COMPONENT MOUNTED"
   // Console WILL print "COMPONENT MOUNTED"