What Is Different Between Null And Undefined?

JavaScript has two cousins in the primitive-value family: null and undefined. Both signal "no value," but they come from different places. Think of undefined as the default state—like a parking spot with no car yet—and null as a deliberate empty space—like a parking spot marked "reserved."

What's Happening

• undefined is what any declared-but-unassigned variable gets by default.
• null is an explicit assignment meaning "no value here."
• Here's a weird quirk: typeof null returns "object"—a historical JavaScript bug that's still around in every browser as of 2026.^{MDN Web Docs}
• Both behave as falsy values, but their actual types don't match—one's an object, the other's its own primitive type.

Step-by-Step Solution

1. Open your browser's developer tools (F12 or Ctrl+Shift+I / Cmd+Opt+I).
2. In the Console tab, type typeof undefined === typeof null and hit Enter.
   You'll see false because their types are different.
3. Type null == undefined (double equals) and press Enter.
   You'll see true because JavaScript performs type coercion.
4. Type null === undefined (triple equals) and press Enter.
   You'll see false because the types don't match exactly.

If This Didn't Work

• Check your scope: if a variable wasn't hoisted into your current block, it'll likely be undefined rather than null.
• Examine your build tools: some transpilers or minifiers accidentally swap undefined for null or vice-versa.
• Test in strict mode: add "use strict"; at the top of your script to prevent accidental globals that muddy these waters.^{MDN Strict Mode}

Prevention Tips

Honestly, this distinction trips up even experienced developers in 2026. Keep null intentional and undefined accidental in your code, and you'll save yourself from a whole category of frustrating bugs.

When does null get used?

Imagine you're building a user profile system. When a user deletes their avatar, you wouldn't leave the field as undefined—you'd set it to null to show you've intentionally removed the image. That's the kind of deliberate choice null is meant for.

When does undefined get used?

This happens all the time in real code. You declare a variable at the top of your function, then forget to initialize it until later. Until that assignment happens, undefined is what you get. It's JavaScript's way of saying, "Hey, nobody put anything here yet."

Why does typeof null return object?

This quirk has stuck around for decades now. Modern JavaScript engines still return "object" for null, even though it's technically incorrect. It's one of those legacy behaviors that's too ingrained to change at this point.^{ECMAScript Language Specification}

How to check for null?

Here's why this matters: value == null would also catch undefined, which isn't what you want if you're specifically looking for null. Be precise with your checks to avoid unexpected behavior.

How to check for undefined?

Now, you might wonder why we don't just use if (!value). That works for falsy values, but it would also catch 0, false, and empty strings—values you might actually want to keep. Be specific in your checks.^{MDN undefined}

How to check for either null or undefined?

This is cleaner than the old-school || operator, which would also replace falsy values like 0 or false. The nullish coalescing operator (??) is purpose-built for this exact scenario.

What about loose equality (==) vs strict equality (===)?

In most cases, you want strict equality (===) because it respects types. But there are legitimate times when you want loose equality (==)—like when checking if a value is either null or undefined. Just be aware of the difference.^{MDN Equality Comparisons}

How do other languages handle this?

JavaScript's approach with both null and undefined is actually pretty rare. Most languages pick one or the other, which makes JavaScript's dual system particularly confusing for developers coming from other backgrounds.^{Wikipedia Null Pointer History}

What Is Different Between Null And Undefined?