Methods, Functions and Prototypes in JavaScript

JavaScript builds objects from functions. Methods, constructors, and prototypes are three ways to attach behavior to objects — and they build on each other.

Methods

A method is a property that holds a function value. That is it — nothing more:

let rabbit = {};
rabbit.speak = function (line) {
  console.log(`The rabbit says '${line}'`);
};

rabbit.speak("I'm alive.");
// → The rabbit says 'I'm alive.'

The function is stored on the object. You call it through the object. That makes it a method.

`this` Binds Methods to Their Object

When you call a method, this refers to the object it was called on. This lets the same function behave differently depending on which object uses it:

function speak(line) {
  console.log(`The ${this.type} rabbit says '${line}'`);
}

let whiteRabbit = { type: "white", speak };
let hungryRabbit = { type: "hungry", speak };

whiteRabbit.speak("Oh my ears and whiskers, how late it's getting!");
// → The white rabbit says 'Oh my ears and whiskers, how late it's getting!'

hungryRabbit.speak("I could use a carrot right now.");
// → The hungry rabbit says 'I could use a carrot right now.'

Same function. Different this. Different output.

Constructor Functions

Writing out Object.create() and assigning properties by hand gets tedious. A constructor function automates this — you call it with new, and JavaScript creates the object, binds this, and returns it for you:

function Rabbit(type) {
  this.type = type;
}

let weirdRabbit = new Rabbit("weird");
console.log(weirdRabbit.type); // → "weird"

The new keyword does four things:

Creates a new empty object
Sets its prototype to Rabbit.prototype
Calls Rabbit with this pointing to the new object
Returns the object (unless the function explicitly returns something else)

Prototypes

Every function in JavaScript has a prototype property. When you create an object with new, that object inherits from the function's prototype. You can add methods to the prototype and every instance gets them:

function Rabbit(type) {
  this.type = type;
}

Rabbit.prototype.speak = function (line) {
  console.log(`The ${this.type} rabbit says '${line}'`);
};

let bigRabbit = new Rabbit("big");
bigRabbit.speak("I am large.");
// → The big rabbit says 'I am large.'

speak lives on Rabbit.prototype, not on each instance. All rabbits share it — saving memory and keeping behavior in one place.

How They Connect

Concept	What it does
Method	A function stored on an object
Constructor	A function called with `new` to create objects
Prototype	An object that other objects inherit methods from

Methods give objects behavior. Constructors create objects with shared setup. Prototypes let those objects share methods without duplicating them.

Rule: if you find yourself copying the same function onto multiple objects, put it on the prototype instead.

Next steps: once you understand prototypes, explore classes — the modern syntax that wraps constructors and prototypes into cleaner code — and arrow functions — which handle this differently.