null is the value that means "there is no value"

Q: What is the sound of one hand clapping?

A: null

Null is useful

Null is used in cases where "nothing yet" is a valid scenario.

For instance, if a user has an account, but doesn't (yet) have a profile picture, account.profilePic may be null.

Then you can test for that case, e.g.

Control Flow Diagram

Null is dangerous

let fruit = 'apple'



fruit = null

Please watch the instructor type this (or type it yourself) and then...

Read the error!

TypeError explained

TypeError: Cannot read property 'toUpperCase' of null
  • "TypeError" means "this is an error about data types" -- you thought you were using a string, but you weren't -- you were actually using null which is not a string

  • "Cannot read property 'toUpperCase'" means "you asked the value for a property named toUpperCase but there was no such property"

  • of null means "the value you were using was null"

This error can be confusing, because it doesn't tell you the name of the variable that was null.

JavaScript has several "empty" values

  • null
  • undefined
  • NaN
  • ''
  • 0
  • false

Docs: MDN: null

This proliferation of nulls is generally a bad idea in language design, but JavaScript is stuck with them.


The null keyword in Javascript means nothing. It is the language's way of saying "there is no value." The null keyword is often put in to a program by the programmer to stand in for a value that will be set later.


undefined is JavaScript's way of saying "I don't know." This is probably the most common empty value, and can creep into your program in a number of ways. Accessing a property that doesn't exist on an object, and trying to call a variable name which hasn't been defined yet are both common ways of getting an undefined value. Functions with no return statement return undefined by default


NaN means "not a number" and is the value you get when you try and convert a type to a number that can not be parsed as a number. Any arithmetic equation that NaN is a part of will evaluate to NaN as any number when combined with NaN is no longer a valid number.

Empty String

The empty string '' is a string without any chaarcters at all in it. A common way for it to get into a program is as a value from a form section that has not been filled out.

Note: A string conataining only white space " " is not an empty string and will evaluate as true since all strings are truthy


The number 0, like in mathematics, is an empty value. It is a number that has no value, and so is treated as an empty value in JavaScript.


The keyword false is a boolean value that means "not true." It is usually not set directly, but is rather the evaluation of an expression,although there are some instances where you may want to specifically set a variable or property to false.

Emptyness is falsy

All of the empty values in the previous slides are falsy, so they will cause an if statement to fall through. This allows the code from the earlier example to be written concisely:

Gaurd Clause Diagram

If user.profilePic is either null or undefined (or false or NaN or '', or 0) then we will show the default picture, thus avoiding a null pointer error when trying to show user.profilePic.

The pattern of checking for a valid value before proceeding is sometimes called a guard clause or defensive programming.

Note the string versions of empty values ("false", "0", "NaN", etc.) are not falsy as they are not empty strings, and are no longer keywords. Anything wrapped in quotations is just a string, nothing more, nothing less.