Null

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.

if (account.profilePic === null) {
    showDefaultPicture();
} else {
    showPicture(account.profilePic);
}

Null is dangerous

let fruit = null
fruit.toUpperCase()

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

Read the error!

Errors are good

They tell you

• you made a mistake
• what that mistake was
• (sometimes) how to fix it

Please try to interpret this error:

fruit.toUpperCase()
TypeError: Cannot read property 'toUpperCase' of null

TypeError explained

fruit.toUpperCase()
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 is confusing because it buries the lede -- you must read all the way to the end before you find the relevant clue ("null"), and it omits the name of the variable whose value was null ("fruit").

Sadly, it is your job as a programmer to translate "TypeError: Cannot read property 'toUpperCase' of null" into "We expected the variable fruit to contain a string, but it contained null instead."

null pointer errors

• null pointer errors are very common
• the trick is reading the error and figuring out
  1. where it happened (which line)
  2. which variable was null
  3. why it was null
• often once you know which, knowing why is obvious
  • but sometimes it's a puzzle and you have to trace back
  • e.g. the original problem was when fruit was set to null, but the error happened later, when the program tried to use null as if it were a string

If You're Going To Fail...

Two failure recovery philosophies:

• fail fast, fail hard
• keep calm and carry on

Which idea is better?

Why or why not?

Failure Recovery

graceful - generally good for users

• provide information and context in non-technical language
• help user accomplish their goal
• allow user to try again immediately

fail-fast - generally good for coders

• exposes errors early
• forces you to think through "rainy day" scenarios
• provides information about the state of the program at the moment the error happened

JavaScript has several nulls

• null means "nothing"
• undefined means "i don't know"
• NaN means "not a number"
• '' means "an empty string"

Docs: MDN: null

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

Nulls are falsy

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

if (user.profilePic) {
    showPicture(user.profilePic);
} else {
    showDefaultPicture();
}

If user.profilePic is either null or undefined (or false or NaN or '') 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.