null vs undefined in JavaScript: The Tale of Two Nothing

🧩 What's the Deal with null and undefined?

In JavaScript, we have two ways to say “nothing” — but they’re not the same.

Both null and undefined represent absence, but they serve very different purposes:

🔍 The Quick Difference

🚀 Common Use Cases

✅ When to Use undefined:

1. Uninitialized variables

let a;
   console.log(a); // undefined

1. Missing function arguments

function greet(name) {
     console.log(name);
   }
   greet(); // undefined

1. Non-existent object properties

const user = {};
   console.log(user.age); // undefined

✅ When to Use null:

1. Intentional absence of a value

let selectedUser = null;

1. Resetting a variable

input.value = null;

1. End of a data structure

const node = { value: 5, next: null };

1. Missing elements in DOM

const el = document.querySelector('.missing'); // null

🔥 Why Does null Even Matter?

Let’s say you’re sending user data:

// Using undefined
{ "middleName": undefined }  // gets stripped out in JSON.stringify

// Using null
{ "middleName": null }       // preserved: "middleName": null

✅ null = "We asked, and they don’t have a middle name."

❌ undefined = "We don’t even know if they have one."

This difference matters in:

• API contracts
• Database schemas
• Validation logic
• Conditional rendering

🤯 Gotchas and Fun Facts

• null == undefined → true (loose equality)
• null === undefined → false (strict equality)
• typeof null → 'object' (weird, but true)

💡 TL;DR: When to Use What?

💬 Interview-Ready Quote:

"undefined is what JavaScript gives you.

null is what you give JavaScript."