Quiz Questions/JS

Explain how prototypal inheritance works

High
JS

This is an extremely common JavaScript interview question. All JavaScript objects have a __proto__ property with the exception of objects created with Object.create(null), that is a reference to another object, which is called the object's "prototype". When a property is accessed on an object and if the property is not found on that object, the JavaScript engine looks at the object's __proto__, and the __proto__'s __proto__ and so on, until it finds the property defined on one of the __proto__s or until it reaches the end of the prototype chain. This behavior simulates classical inheritance, but it is really more of delegation than inheritance.

Example of Prototypal Inheritance

function Parent() {
this.name = 'Parent';
}
Parent.prototype.greet = function () {
console.log('Hello from ' + this.name);
};
const child = Object.create(Parent.prototype);
child.cry = function () {
console.log('waaaaaahhhh!');
};
child.cry();
// waaaaaahhhh!
child.greet();
// hello from Parent
child.constructor;
// ƒ Parent() {
// this.name = 'Parent';
// }
child.constructor.name;
// 'Parent'

Things to note are:

  • .greet is not defined on the child, so the engine goes up the prototype chain and finds .greet off the inherited from Parent.
  • We need to call Object.create in one of following ways for the prototype methods to be inherited:
    • Object.create(Parent.prototype);
    • Object.create(new Parent(null));
    • Object.create(objLiteral);
    • Currently, child.constructor is pointing to the Parent:

If we'd like to correct this, one option would be to do:

function Parent() {
this.name = 'Parent';
}
Parent.prototype.greet = function () {
console.log('Hello from ' + this.name);
};
function Child() {
Parent.call(this);
this.name = 'Child';
}
Child.prototype = Object.create(Parent.prototype);
Child.prototype.constructor = Child;
const child = new Child();
child.greet();
// hello from Child
child.constructor.name;
// 'Child'

References