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Programming Fundamentals › Programming Basics

undefined

JavaScript's value for a variable or property that was never assigned.

Frontend Engineer · Junior · core

Also known as: undefined value, typeof undefined

undefined is what JavaScript gives you when there’s no value because nothing was ever put there. You rarely write it yourself; you mostly meet it:

let x;                        // declared, never assigned → undefined
const user = { name: "Ana" };
user.email;                   // property doesn't exist → undefined
const items = [1, 2, 3];
items[10];                    // out of range → undefined
function greet() {}           // no return statement
greet();                      // → undefined
function hello(name) { return name; }
hello();                      // missing argument → undefined

undefined vs null

Both mean “no value”, but by convention:

  • undefined: the language says “nothing was set here”.
  • null: you say “this is intentionally empty”.
null == undefined;    // true  (loose equality treats them alike)
null === undefined;   // false (they are different values)
typeof undefined;     // "undefined"

JSON has no undefined: JSON.stringify({ a: undefined }) gives "{}", so the key disappears, while null survives.

Where it bites

The famous crash, “Cannot read properties of undefined”, happens when you go one level too deep:

const user = {};
user.address.city;       // TypeError: user.address is undefined
user.address?.city;      // undefined, no crash

Optional chaining (?.) avoids the crash, but ask why the value was missing before hiding it. Often the real cause is data that hasn’t loaded yet, or a typo in a property name.

Default parameters apply for undefined only, not for null: function f(x = 5) gives 5 for f() and f(undefined), but null for f(null).

Rule of thumb: don’t assign undefined on purpose. Use null when you need to say “empty”, and check with === undefined or == null depending on whether you want to match one or both.