null vs undefined | Engineering Interview Kit

TL;DR

undefined: Means a variable has been declared but has not yet been assigned a value. It’s JavaScript’s default “empty” state.
null: Means “nothing” or “empty”. It is explicitly assigned by the developer to indicate that a variable intentionally holds no value.

Mental Model

How It Works

undefined is assigned by the JavaScript engine in many scenarios:

A declared variable with no value (let x;).
A function that doesn’t explicitly return anything.
Accessing an object property that doesn’t exist (user.fakeProperty).
Accessing an array index out of bounds (arr[99]).

null is never automatically assigned by JavaScript. You must type = null. It is often used to clear a variable’s contents for Garbage Collection, or as a default starting state for an object that will be fetched later.

The typeof Bug:

typeof undefined is "undefined".
typeof null is "object". (This is a famous bug from the very first version of JavaScript, but fixing it would break the internet, so it remains forever).

Example

// --- undefined ---
let username;
console.log(username); // undefined (System set)

function doNothing() {}
console.log(doNothing()); // undefined (Implicit return)

const user = { name: "Alice" };
console.log(user.age); // undefined (Property doesn't exist)

// --- null ---
let activeUser = null; // Explicitly stating there is no active user yet
console.log(activeUser); // null

Common Output Question

console.log(Number(null));
console.log(Number(undefined));

Q: What does this output and why?
A:
0
NaN
When doing math, JavaScript coerces null into 0. However, undefined is converted into NaN (Not a Number). This makes undefined slightly more dangerous in math operations because NaN poisons any subsequent calculations.

Senior Interview Question

Q: How do you safely check if an object property exists without triggering a false positive if the property is explicitly set to undefined?

A: If you use a simple truthy check if (user.age), it fails if age is 0, "", or undefined.
If you use if (user.age !== undefined), it fails if the developer explicitly set user.age = undefined.
To truly check if the key exists on the object, regardless of its value, you must use the in operator or hasOwnProperty:
if ("age" in user) or if (Object.hasOwn(user, "age")).