How do you check if a property exists?

There are four common ways, and they answer slightly different questions.

const obj = { a: 1, b: undefined };

"a" in obj;                          // true  — own or inherited
Object.hasOwn(obj, "a");             // true  — own only
obj.hasOwnProperty("a");             // true  — own only, older form
obj.a !== undefined;                 // true  — but see below

The difference that matters

in walks the prototype chain, so it reports inherited properties too:

"toString" in obj;                   // true  — inherited from Object.prototype
Object.hasOwn(obj, "toString");      // false — not the object's own

For a plain data object that distinction rarely matters; for anything with a prototype, or when checking keys from user input, it matters a lot.

Why comparing to undefined is not the same

A property can exist and hold undefined:

obj.b !== undefined;                 // false — but the key IS there
Object.hasOwn(obj, "b");             // true

That difference decides whether { b: undefined } means "not provided" or "explicitly cleared" — for example when building a partial update.

Prefer Object.hasOwn

It replaces hasOwnProperty and avoids two of its problems: an object can shadow the method with its own hasOwnProperty key, and objects made with Object.create(null) do not have it at all. The old workaround was Object.prototype.hasOwnProperty.call(obj, key) — Object.hasOwn(obj, key) says the same thing plainly.

Related tools

Optional chaining reads safely without checking first: obj?.a?.b. And ?? distinguishes "missing or null" from falsy values, so obj.count ?? 10 keeps a legitimate 0 that obj.count || 10 would discard.