Learning Objective: Differentiate between and values and understand their unique roles in JavaScript.

What is ?

is a special value in JavaScript used to represent “nothing,” “empty,” or “value unknown.” It is intentionally set by a programmer to indicate that a variable should have no value.

Example: Assigning

let age = null; // This means "age is unknown or empty."
console.log(age); // null

Key Points:

• is a primitive value in JavaScript.
• It is not an object (although typeof null returns "object" due to a historical quirk).

What is ?

is the default value for variables that have been declared but not assigned a value.

Example: Uninitialized Variables

let x; // No value assigned
console.log(x); // undefined

Can you assign ?

While it’s technically possible, it’s not recommended to assign manually. Use for this purpose instead.

let x = 123;
x = undefined; // Not recommended
console.log(x); // undefined

Key Differences Between and

Understanding the Behavior

1. means a variable exists but doesn’t have a value:

   let name; // Declared but not assigned
   console.log(name); // undefined

2. means the variable intentionally has no value:

   let name = null; // Explicitly set to "no value"
   console.log(name); // null

3. They are not strictly equal:

   console.log(null === undefined); // false

4. But they are loosely equal:

   console.log(null == undefined); // true

Using to Inspect Types

You can inspect the type of any value using :

let empty = null;
let notSet;

console.log(typeof empty); // "object" (a JavaScript quirk!)
console.log(typeof notSet); // "undefined"

Interactive Activity

1. Open VS Code and create a file called .
2. Write the following:
   • Declare a variable with to represent “no value.”
   • Declare another variable without assigning a value to make it .
   • Use to inspect the type of both variables.
   • Log the results to the console.

Example:

let empty = null; // null value
let notSet; // undefined

console.log(empty); // null
console.log(typeof empty); // "object"

console.log(notSet); // undefined
console.log(typeof notSet); // "undefined"

1. Change the value of the variable to something else and observe the changes.

What’s Next?

In this lesson, you learned the difference between and . These concepts are foundational for debugging and writing clear, intentional code. Practice working with them, and don’t forget to explore their behavior using !

Next, we’ll explore objects, a key data type in JavaScript that enables you to store and manage collections of data. Let’s keep going! 🚀

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