hasOwnProperty vs in

The in operator and hasOwnProperty function are the common ways to check if an objects contains a particular key.

const person = {
    name: 'Foo',
};

'name' in person; // true
person.hasOwnProperty('name'); // true

Differences

1. For inherited properties, in will return true. hasOwnProperty, as the name implies, will check if a property is owned by itself, and ignores the inherited properties.

   Let's reuse the person object from the previous example. Since it's a JavaScript object which has built-in properties such as constructor, __proto__, the following check return true:

   'constructor' in person; // true
   '__proto__' in person; // true
   'toString' in person; // true

   While hasOwnProperty returns false when checking against these properties and methods:

   person.hasOwnProperty('constructor'); // false
   person.hasOwnProperty('__proto__'); // false
   person.hasOwnProperty('toString'); // false

2. For the get and set methods of a class, hasOwnProperty also returns false.

   For example, we have a simple class representing triangles:

   class Triangle {
       get vertices() {
           return 3;
       }
   }
   
   // Create new instance
   const triangle = new Triangle();

   Despite the fact that vertices is the property of triangle:

   triangle.vertices; // 3
   'vertices' in triangle; // true

   hasOwnProperty still ignores it:

   triangle.hasOwnProperty('vertices'); // false

Good practice

In order to check the existence of a property, use hasOwnProperty. Use in to check the existence of a method.

See also

• Object vs Map