Representing "Nothing"

Representing the lack of a value is confusing in JavaScript since there are two different ways to do it: null and undefined.

What Are null and undefined?

Both null and undefined represent the lack of a value, but they're used in different situations:

• undefined - means a variable exists but hasn't been given a value yet
• null - means you've intentionally set a variable to have no value

Understanding undefined

When you create a variable but don't give it a value, JavaScript automatically sets it to undefined:

Copylet a
console.log(a) // undefined

There are other cases where you might see undefined, which will be covered later, but it always means a value does not exist.

Understanding null

null is a value used to indicate "intentionally empty":

Copylet a = null
console.log(a) // null

By setting a value to null, you are explicitly saying this variable has no value.

Best Practices

1. Use null for Intentional "Empty" States

   Copylet selectedFile = null
   let currentTheme = null
   let lastError = null
   

2. Let undefined Happen Naturally

   Copylet userName // Will be set later
   let userPreferences // Will be loaded from server
   

3. Use null to Clear Values

   Copylet name = "Kyle"
   // Other code uses name...
   name = null // Now name has no value
   

The Key Difference

• undefined - I haven't been given a value yet
• null - I specifically have no value