Null & Undefined

Undefined

Undefined means "a value hasn't been assigned yet." Think of it as an empty spot in memory that exists but hasn't been filled with anything.

It occurs when:

• You declare a variable but don't assign it a value
• You try to access an object property that doesn't exist
• A function doesn't explicitly return anything
• You have a function parameter that wasn't provided

let dog;  // declared but not initialized - value is undefined
console.log(dog);  // undefined

function bark() {
  // no return statement
}
console.log(bark());  // undefined

const pet = {};
console.log(pet.name);  // undefined (this property doesn't exist)

Null

Null means "intentionally empty or nothing." It's a deliberate assignment to represent "no value" or "empty value."

Unlike undefined, null is always explicitly assigned:

let cat = null;  // I deliberately set this to "nothing" for now

Key Differences to Remember

Think of it like this:

• Undefined is like an empty shelf that exists but has nothing on it yet (JavaScript assigns this automatically)
• Null is like putting a box labeled "EMPTY" on the shelf (you have to put it there yourself)

More technically:

• typeof undefined is "undefined"
• typeof null is "object" (this is actually a historical bug in JavaScript!)
• null == undefined is true (loose equality)
• null === undefined is false (strict equality)