in

Examples

Does the property exist:

const user = { name: 'Anna', age: undefined };
 
console.log('name' in user);
console.log('email' in user);
console.log('age' in user);

Output:

true
false
true

Inherited properties count too:

console.log('toString' in {});
console.log(Object.hasOwn({}, 'toString'));

Output:

true
false

Notes

Do not confuse it with the for...in loop — that is a different construct. For arrays in checks indexes, not values: 2 in ['a', 'b', 'c'] is true. Search array values with includes().