JavaScript undefined and null: What Is the Difference?

JavaScript has two ways of saying “there is no value here”. They are not the same.

undefined: nobody gave it a value

JavaScript uses undefined when something has not been set. You will meet it often:

script.js

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

const user = { name: "Maya" };
console.log(user.email);      // → undefined (no such property)

function greet(name) {
  console.log(name);          // → undefined (no argument was passed)
}
greet();

function noReturn() {}
console.log(noReturn());      // → undefined (nothing returned)

null: someone chose “empty”

null is a value you assign to say “this is intentionally empty”.

script.js

let selectedUser = null;   // nobody selected yet
console.log(selectedUser); // → null

selectedUser = { name: "Maya" };

A common use: an API returns null when it knows there is no result.

Comparing them

script.js

console.log(undefined === null); // → false
console.log(undefined == null);  // → true (loose equality treats them as equal)
console.log(typeof undefined);   // → undefined
console.log(typeof null);        // → object

The last line is a famous historical bug: typeof null returns "object", but null is a primitive. It cannot be fixed without breaking old websites.

Checking for “no value”

script.js

function describe(value) {
  if (value === undefined) return "undefined";
  if (value === null) return "null";
  return "has a value";
}

console.log(describe());       // → undefined
console.log(describe(null));   // → null
console.log(describe(0));      // → has a value

Notice that 0 counts as a value. Do not use a plain if (!value) when 0 or "" are valid. The operator ?? in Nullish Coalescing handles exactly this case.

lightbulb

Rule of thumb

Let JavaScript produce undefined. Use null yourself only when you want to say “empty on purpose”.