Dmitri Pavlutin
Thoughts on Frontend development

Own and Inherited Properties in JavaScript

Posted May 24, 2020

In JavaScript, contrary to other programming languages like Java or Python, there’s no template (e.g. class) concept for creating objects.

Every JavaScript object links to another object named prototype, from which the object inherits properties.

In this post, I’ll describe the difference between own and inherited properties. It’s a good way to understand the slightly unusual inheritance mechanism of JavaScript.

1. Own properties

An own property is a property defined directly on the object.

Let’s define a plain JavaScript object with one property:

const myObject = {
  myProp: 'Value'
};

myObject.myProp; // => 'Value'

myObject is a plain JavaScript object. The property myProp is defined directly on myObject, being an own property.

To list the own properties of an object use the built-in utility function Object.getOwnPropertyNames(object).

Let’s list the own properties of myObject:

const myObject = {
  myProp: 'Value'
};

Object.getOwnPropertyNames(myObject); // => ['myProp']

Object.getOwnPropertyNames(myObject) returns an array having one own property name: ['myProp'].

2. Inherited properties

An inherited property is a property the object inherits from the prototype object.

Every object in JavaScript links to an object, the prototype, from which it inherits properties.

Let’s use again myObject. This time let’s access a property that you haven’t defined upon myObject directly:

const myObject = {
  myProp: 'Value'
};

myObject.toString; // => function() {...}

The property accessor myObject.toString evaluates to a function.

toString is an inherited property. In other words, myObject inherits toString property from its prototype object.

When JavaScript evaluates the expression myObject.toString, first, it tries to find the property toString within the own properties - however it cannot find one (myObject has just one own property myProp). Then JavaScript looks inside the prototype object of myObject, and finally finds a property toString.

Inherited toString property of myObject equals to the same property access directly from the prototype object:

const myObject = {
  myProp: 'Value'
};

const myObjectProto = Object.getPrototypeOf(myObject);

myObject.toString === myObjectProto.toString; // => true

Where Object.getPrototypeOf(object) is an utility function that returns the object’s prototype.

3. Prototype as a source of inherited properties

When I was trying to understand the prototypal inheritance in JavaScript, I was thinking that the prototype object is a complex or special God object. But it’s much simpler.

Think about the prototype object as a source of inherited properties for an object.

4. Own vs inherited

Let’s slightly modify myObject and define a method toString directly on it:

const myObject = {
  myProp: 'Value',
  toString() {
    return `[object MyObject]`;
  }
};

const myObjectProto = Object.getPrototypeOf(myObject);

myObject.toString === myObjectProto.toString; // => false

Because myObject has an own property toString, the object does no longer inherit toString from the prototype object.

When an object has an own property and inherits a property with the same name, the own property takes precedence over the inherited one.

If an own property gets deleted, then the inheritance re-activates:

const myObject = {
  myProp: 'Value',
  toString() {
    return `[object MyObject]`;
  }
};

// Own properties
myObject.toString(); // => '[object MyObject]'
myObject.myProp;     // => 'Value'

delete myObject.toString;
delete myObject.myProp;

// Inherited property
myObject.toString(); // => '[object Object]'

// No inherited property
myObject.myProp;     // => undefined

The first method invocation myObject.toString() uses the own property. Then delete myObject.toString deletes the own property.

The second invocation myObject.toString(), even having the own property toString deleted, uses the inherited toString property from the prototype object.

However, there’s no myProp inherited from the prototype. When the own prop myProp is deleted from the object delete myObject.myProp, later the expression myObject.myProp evaluates to undefined.

5. Summary

A JavaScript object can have either own or inherited properties.

The own property means that the property is defined directly on the object. On the other side, the inherited property is the one inherited from the prototype object.

Knowing the difference between own and inherited properties is a big step in understanding the prototypal inheritance of JavaScript.

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About Dmitri Pavlutin

I'm a passionate software developer, tech writer and coach. My daily routine consists of (but not limited to) drinking coffee, coding, writing, coaching, overcoming boredom 😉.