/this-vs-that GitHub 1316★

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


  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

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