How can I check whether an object has its own property?

Use Object.hasOwn(object, key). It returns true for a property directly on the object and false for one found only through the prototype chain. By contrast, key in object checks both the object and its prototype chain. [S1] [S2]

Here is a small example with one own property and one inherited property:

const proto = { inherited: 2 };
const item = { own: 1 };
Object.setPrototypeOf(item, proto);

console.log(Object.hasOwn(item, "own"));       // true
console.log(Object.hasOwn(item, "inherited")); // false
console.log("own" in item);                    // true
console.log("inherited" in item);              // true

The choice depends on what “has” means for your task: direct ownership, or availability anywhere along the prototype chain.

For the item example above, the results are:

• Object.hasOwn(item, "own") → true
• "own" in item → true
• Object.hasOwn(item, "inherited") → false
• "inherited" in item → true

Object.hasOwn() asks whether the property is directly on item. The in operator asks whether it is on item or anywhere in its prototype chain. [S1] [S2]

So if you are filtering an object’s directly defined fields, use Object.hasOwn(). If inherited properties should count as present, use in. The checks do not tell you the property's value; they answer whether the property is present under their respective rules.

A property can be present even when its value is undefined. The in operator still returns true in that case; presence is different from checking whether a value is defined. [S2] Object.hasOwn() likewise checks whether the property is directly on the object, including when its value is null or undefined. [S1]

For example:

const item = { note: undefined };

Object.hasOwn(item, "note"); // true: own property
"note" in item;              // true: property is present
item.note !== undefined;     // false: value is not non-undefined

If your requirement is specifically “does this object directly define note?”, use Object.hasOwn(). If it is “is note present on this object or its prototype chain?”, use in. If your requirement is about the value, check that value separately.

Use this small helper when the question is ownership:

function hasOwn(object, key) {
  return Object.hasOwn(object, key);
}

const hasOwnName = hasOwn(item, "name");
const nameAvailable = "name" in item;

Choose Object.hasOwn() when you need a property directly on the object. Choose in when an inherited property should count. Remember that the right-hand operand of in must be an object; using a primitive there throws a TypeError. [S2] These checks do not establish that a property’s value has a particular type or meaning, so validate the value separately when needed.