Sometimes you've gotta check to make sure that nothing isn't actually...nothing. 😲❗❓

In JavaScript, null is a primitive type intentionally containing the value of null. Undefined is a primitive type and represents a variable you declare without initiating a value.

So, null is nothing and undefined is just missing something. 🤣

nothing
null is nothing; undefined is not something

Not super helpful, I know. Let's dive deeper.

How to Define Null and Undefined Values

An example will help. Below we declare two variables. Let's keep it simple and use null and undefined to compare outcomes since they are sometimes confused because of their similarities.

let leviticus = null;
// leviticus is null

let dune;
// dune is undefined
Declaring two variables: one null and one undefined.

leviticus is intentionally without an object value (null). Whereas dune is declared, yet it's unintentionally missing a value (undefined).

How to Check for Null with typeof()

You can check for null with the typeof() operator in JavaScript.

console.log(typeof(leviticus))
// object

console.log(typeof(dune))
// undefined
typeof() will return 'object' when called on a null variable

Curiously, if you check with typeof(), a null variable will return object. This is because of a historic bug in JavaScript.

How to Check for Null with Equality Operators

Another curiosity is that when you loosely check for equality using double equals ==, null and undefined will return true.

console.log(leviticus == dune)
// true

console.log(leviticus === dune)
// false

console.log(leviticus == null)
// true (but not as good a habit to use as strict equality shown in next example)

But when you strictly check for equality using triple equals ===, null and undefined will return false.

This is because null and undefined are both falsy in JavaScript. Falsy means that a value is considered false when encountered in a Boolean (true or false) context.

JavaScript uses coercion to coerce values from one type to another in order to be able to use them in a Boolean context.

But by strictly checking for equality, you can see that they are in fact not equal.

How to to Check for Null with Strict Equality

The best way to check for null is to use strict and explicit equality:

console.log(leviticus === null)
// true

console.log(dune === null)
// false

How to Check for Null with the Object.is() Method

An equally foolproof way to check for null is to use the built-in Object.is() method:

console.log(Object.is(leviticus, null)
// true
            
console.log(Object.is(dune, null)
// false

Summary

  • null is a primitive type of a variable which evaluates falsy, has a typeof() of object, and is typically intentionally declared as null
  • undefined is a primitive type of a variable which evaluates falsy, has a typeof() of undefined, and represents a variable that is declared but missing an initial value.
  • null == undefined evaluates as true because they are loosely equal.
  • null === undefined evaluates as false because they are not, in fact, equal.
  • <null_variable> === null is the best way to strictly check for null.
  • Object.is(<null_variable>,null) is an equally reliable way to check for null.

Take heart! As you've probably gathered, there are a plethora of brain teasers in the JavaScript ecosystem like this. But when you break it down, you can confidently understand them/

denzel
You can do it!

Thanks for Reading!

I hope this was a helpful breakdown for you. Keep coding, and keep leaning forward!

Come say hey to me over on Twitter: https://twitter.com/EamonnCottrell

Have a great one 👋.