Prototype Chain in JavaScript: How Inheritance Really Works

You’ve probably written JavaScript code that worked… but the moment you tried to access a method or property you never created yourself, things got confusing fast.

Maybe an object returned a value you didn’t expect, or you wondered how built-in methods like .push() or .toString() magically exist on your objects.

At some point, every JavaScript learner hits this wall — and it usually leads to the same question: How does JavaScript actually find these properties?

This is where the prototype chain comes in.

It’s one of the core ideas behind how JavaScript works under the hood, and understanding it makes the language feel far more predictable and easier to debug.

What Is the Prototype Chain in JavaScript?

The prototype chain is the system JavaScript uses to share properties and methods between objects.

Instead of copying methods into every object, JavaScript links objects together through a series of hidden connections called prototypes.

When you try to access a property on an object, JavaScript will:

1. Check the object itself.
2. If it doesn’t find it, check the object’s prototype.
3. If it’s not there either, check the prototype’s prototype.
4. Continue searching upward until it either finds the property or reaches the end of the chain (null).

This upward “property lookup path” is what we call the prototype chain.

In simpler terms, imagine every object has a backpack. If it can’t find something in its own backpack, it looks in the next backpack it’s connected to, and so on.

The full line of backpacks is the prototype chain.

This chain is what makes JavaScript’s inheritance system work. Methods like .toString(), .push(), .map(), or .hasOwnProperty() exist on higher-level prototypes, and your objects automatically inherit them through this chain instead of having them defined directly on each object.

Why Does the Prototype Chain Exist in JavaScript?

The prototype chain exists because JavaScript was designed around shared behavior instead of duplicated behavior.

It’s the underlying system that allows objects to inherit functionality in a simple, memory-efficient, and predictable way.

To Enable Inheritance Without Classical Classes

Unlike languages such as Java or C#, early JavaScript didn’t use traditional classes.

Instead, it let objects inherit directly from other objects, following a model called prototypal inheritance.

The prototype chain is how JavaScript implements this:

• One object can link to another.
• That object can link to another.
• This creates a simple chain where behavior flows “upward.”

Even though ES6 introduced class, the class system still uses this same prototype chain underneath.

To Share Methods Efficiently (Better Memory Usage)

Without prototypes, every object would need its own copy of built-in methods like:

• .toString()
• .hasOwnProperty()
• .map()
• .push()

That would waste a huge amount of memory.

By storing these methods once on a shared prototype, JavaScript lets all objects access them through the prototype chain.

Instead of duplicating functionality, JS reuses it.

Example:

If 10,000 arrays need .push(), they don’t each store a separate version.

They inherit it from Array.prototype through the chain.

To Provide a Default Set of Behaviors for All Objects

Every object in JavaScript automatically inherits from Object.prototype.

This gives all objects a shared base set of “default tools,” like:

• toString()
• valueOf()
• isPrototypeOf()

Without the prototype chain, none of these built in helpers would exist on objects automatically, and you would have to define them yourself.

In short: The prototype chain exists because JavaScript needed a simple way for objects to share behavior, a memory friendly way to reuse methods, and a reliable way to give every object useful tools right out of the box.

How the Prototype Chain in JavaScript Works

The prototype chain works by letting JavaScript look for properties in a specific order.

When you access something on an object, JavaScript checks the object first. If it is not there, it moves up to the object's prototype and keeps going until it finds the property or reaches the end of the chain.

This behavior is what allows objects to reuse methods, inherit features, and share functionality without copying anything.

The Object Is Checked First

When you try to access a property, JavaScript first checks if the object itself owns that property.

If it does, it stops and returns the value.

If It’s Not There, JavaScript Looks at the Prototype

If the property is not found on the object, JavaScript checks its prototype — the object linked through its internal [[Prototype]].

The Search Continues Up the Chain

If the prototype doesn’t have it, JavaScript checks that prototype’s prototype, and so on.

This continues until a match is found or the chain reaches null.

The Chain Ends at Object.prototype

Almost all objects eventually link back to Object.prototype.

Once JavaScript reaches a prototype whose own prototype is null, the search stops.

If the property still isn’t found, the result becomes undefined.

Shadowing Can Override What the Chain Finds

If an object has its own property with the same name as one found higher in the chain, the own property takes priority.

This is known as shadowing.

Prototype Chain Example (With Diagram)

Now that you know how the prototype chain works in theory, it helps to see it in action.

Below is a simple example that shows how JavaScript walks up the chain when looking for a property. This gives you a clearer picture of what actually happens behind the scenes.

A Simple Prototype Chain‍

const base = { baseValue: "from base" };
const mid = Object.create(base);
mid.midValue = "from mid";

const top = Object.create(mid);
top.topValue = "from top";

console.log(top.topValue);   // found immediately on 'top'
console.log(top.midValue);   // not on 'top' → found on 'mid'
console.log(top.baseValue);  // not on 'top' or 'mid' → found on 'base'
console.log(top.missing);    // not found anywhere → undefined

‍


What JavaScript Does Step by Step

When you ask for top.midValue, JavaScript checks:

1. Does top have midValue?
2. No.
3. Check top’s prototype (mid).
4. Found.
5. Return the value.

When you ask for top.baseValue, JavaScript checks:

1. top → not found
2. mid → not found
3. base → found
4. Return the value

When you ask for something that does not exist:

1. top → no
2. mid → no
3. base → no
4. Object.prototype → no
5. null (end of chain)
6. Result becomes undefined

Visual Diagram of the Chain‍

top
  ↓
mid
  ↓
base
  ↓
Object.prototype
  ↓
null

‍


Each arrow represents the next place JavaScript checks when trying to find a property.

This is the prototype chain in its simplest, most understandable form:

JavaScript keeps looking upward until it finds what you asked for, or until it has nowhere left to look.

Common Mistakes to Avoid With the Prototype Chain

Once you understand how the prototype chain works, the next step is avoiding the common issues it can create. Many JavaScript bugs come from small misunderstandings about inheritance or shadowing.

Here are the mistakes to watch out for so your code behaves as expected.

Adding Mutable Values to a Prototype

Prototypes are shared by all objects that inherit from them.

If you put something like an array or object on a prototype, every instance will use the same shared value.‍

function Item() {}
Item.prototype.tags = [];  // shared by every instance

const a = new Item();
const b = new Item();

a.tags.push("new");
console.log(b.tags); // also shows ["new"]

‍


This often leads to confusing bugs, especially when you expect each instance to have its own copy.

Shadowing Properties Without Realizing It

If an object has its own property with the same name as one on the prototype, the own property always takes priority.

This is known as shadowing.‍

const base = { name: "Base" };
const obj = Object.create(base);

obj.name = "Custom";  // shadows the value on 'base'
console.log(obj.name);  // "Custom"

‍


Shadowing is useful, but it can also hide inherited values you might expect to use.

Accidentally Overwriting a Prototype

Reassigning a prototype breaks the original inheritance chain and often removes built-in methods.

function User() {}
User.prototype = {};  // overwrites the original prototype

‍


This removes the default constructor reference and any methods that may have already existed.

It is usually safer to extend the existing prototype instead of replacing it entirely.

Modifying Built-In Prototypes

Changing things like Array.prototype or Object.prototype affects every piece of code that works with those objects.

This can create unpredictable issues, especially when working in teams or using third-party libraries.

Array.prototype.last = function () { ... };  // risky

‍

It is better to avoid modifying built-in prototypes unless you fully understand the impact.

Relying on Deep Prototype Chains

Deep inheritance chains make property lookup slower and harder to follow.

The more layers JavaScript has to check, the more confusing your code becomes.

Keeping your prototype chains shallow makes your code easier to reason about and debug.

FAQs

How do I inspect the prototype chain in my code?

You can inspect the prototype chain using tools like Object.getPrototypeOf(), browser DevTools, or by checking an object's properties with hasOwnProperty(). These let you see where a property comes from and how the chain is structured.

What are real-world uses of the prototype chain?

Developers rely on the prototype chain to reuse methods, structure inheritance with classes, and create objects that share common behavior. It keeps code more efficient by avoiding duplicate functions across multiple objects.

Is JavaScript prototype-based or class-based?

JavaScript is prototype-based at its core, even though it supports class syntax. Classes in JS still rely on the prototype chain behind the scenes.

Does the prototype chain hurt performance?

A normal prototype chain has little impact on performance. Issues only appear when chains become unnecessarily deep, which makes property lookups take longer.

Can I remove items from the chain?

You cannot remove specific links inside the chain, but you can replace or reset an object’s prototype. The chain ends naturally once a prototype’s value becomes null.

Conclusion

Now that you know how inheritance works and how objects share behavior, you can move forward with a clearer grasp of how the language thinks behind the scenes.

If you are exploring tools to practice and write JavaScript more effectively, you might also find value in reading our Replit review, especially if you prefer learning through active coding.

If you want to keep improving your engineering fundamentals, you can explore more guides on our blog.

If you are preparing for interviews and want structured practice, you can try our company online assessments.

If you are studying higher level engineering concepts, you might find our system design resource helpful.

And if you want personalized guidance for your growth as a software engineer, you can work with our software engineering career coach.

No matter what you choose to focus on next, we wish you the best.

If you want to explore everything we offer in one place, you can always start with Lodely.

‍