Null and Undefined

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Let’s cycle back to primitives. Earlier, you learned that there are seven possible primitives in JavaScript:

We’ll focus on Null and Undefined values in this lesson.

Undefined

undefined is a value that indicates an absence of a value. We say that something is undefined when it is not explicitly defined.

For example, if you declare a variable but did not assign anything to it, the variable will be undefined.

let a
console.log(a) // undefined

If you have a function that returns nothing, the result of the function will be undefined.

const returnsNothing = () => {}

const test = returnsNothing()
console.log(test) // undefined

null is a value that is used to indicate “nothingness”. Developers need to explicitly set a value to be null.

const zell = {
  firstName: 'Zell',
  middleName: null,
  lastName: 'Liew'
}

console.log(zell.middleName) // null

If you compare null and undefined with the strictly equal operator, you’ll get false.

null === undefined // false

This is because null and undefined are different primitives.