How Does React Tell a Class from a Function?

Consider this Greeting component which is defined as a function:

function Greeting() {
  return <p>Hello</p>;
}

React also supports defining it as a class:

class Greeting extends React.Component {
  render() {
    return <p>Hello</p>;
  }
}

(Until recently, that was the only way to use features like state.)

When you want to render a <Greeting />, you don’t care how it’s defined:

// Class or function — whatever.
<Greeting />

But React itself cares about the difference!

If Greeting is a function, React needs to call it:

// Your code
function Greeting() {
  return <p>Hello</p>;
}

// Inside React
const result = Greeting(props); // <p>Hello</p>

But if Greeting is a class, React needs to instantiate it with the new operator and then call the render method on the just created instance:

// Your code
class Greeting extends React.Component {
  render() {
    return <p>Hello</p>;
  }
}

// Inside React
const instance = new Greeting(props); // Greeting {}
const result = instance.render(); // <p>Hello</p>

In both cases React’s goal is to get the rendered node (in this example, <p>Hello</p>). But the exact steps depend on how Greeting is defined.

So how does React know if something is a class or a function?

Just like in my previous post, you don’t need to know this to be productive in React. I didn’t know this for years. Please don’t turn this into an interview question. In fact, this post is more about JavaScript than it is about React.

This blog is for a curious reader who wants to know why React works in a certain way. Are you that person? Then let’s dig in together.

This is a long journey. Buckle up. This post doesn’t have much information about React itself, but we’ll go through some aspects of new, this, class, arrow functions, prototype, __proto__, instanceof, and how those things work together in JavaScript. Luckily, you don’t need to think about those as much when you use React. If you’re implementing React though...

(If you really just want to know the answer, scroll to the very end.)

---

First, we need to understand why it’s important to treat functions and classes differently. Note how we use the new operator when calling a class:

// If Greeting is a function
const result = Greeting(props); // <p>Hello</p>

// If Greeting is a class
const instance = new Greeting(props); // Greeting {}
const result = instance.render(); // <p>Hello</p>

Let’s get a rough sense of what the new operator does in JavaScript.

---

In the old days, JavaScript did not have classes. However, you could express a similar pattern to classes using plain functions. Concretely, you can use any function in a role similar to a class constructor by adding new before its call:

// Just a function
function Person(name) {
  this.name = name;
}

var fred = new Person('Fred'); // ✅ Person {name: 'Fred'}
var george = Person('George'); // 🔴 Won’t work

You can still write code like this today! Try it in DevTools.

If you called Person('Fred') without new, this inside it would point to something global and useless (for example, window or undefined). So our code would crash or do something silly like setting window.name.

By adding new before the call, we say: “Hey JavaScript, I know Person is just a function but let’s pretend it’s something like a class constructor. Create an {} object and point this inside the Person function to that object so I can assign stuff like this.name. Then give that object back to me.”

That’s what the new operator does.

var fred = new Person('Fred'); // Same object as `this` inside `Person`

The new operator also makes anything we put on Person.prototype available on the fred object:

function Person(name) {
  this.name = name;
}
Person.prototype.sayHi = function() {
  alert('Hi, I am ' + this.name);
}

var fred = new Person('Fred');
fred.sayHi();

This is how people emulated classes before JavaScript added them directly.

---

So new has been around in JavaScript for a while. However, classes are more recent. They let us rewrite the code above to match our intent more closely:

class Person {
  constructor(name) {
    this.name = name;
  }
  sayHi() {
    alert('Hi, I am ' + this.name);
  }
}

let fred = new Person('Fred');
fred.sayHi();

Capturing developer’s intent is important in language and API design.

If you write a function, JavaScript can’t guess if it’s meant to be called like alert() or if it serves as a constructor like new Person(). Forgetting to specify new for a function like Person would lead to confusing behavior.

Class syntax lets us say: “This isn’t just a function — it’s a class and it has a constructor”. If you forget new when calling it, JavaScript will raise an error:

let fred = new Person('Fred');
// ✅  If Person is a function: works fine
// ✅  If Person is a class: works fine too

let george = Person('George'); // We forgot `new`
// 😳 If Person is a constructor-like function: confusing behavior
// 🔴 If Person is a class: fails immediately

This helps us catch mistakes early instead of waiting for some obscure bug like this.name being treated as window.name instead of george.name.

However, it means that React needs to put new before calling any class. It can’t just call it as a regular function, as JavaScript would treat it as an error!

class Counter extends React.Component {
  render() {
    return <p>Hello</p>;
  }
}

// 🔴 React can't just do this:
const instance = Counter(props);

This spells trouble.

---

Before we see how React solves this, it’s important to remember most people using React use compilers like Babel to compile away modern features like classes for older browsers. So we need to consider compilers in our design.

In early versions of Babel, classes could be called without new. However, this was fixed — by generating some extra code:

function Person(name) {
  // A bit simplified from Babel output:
  if (!(this instanceof Person)) {
    throw new TypeError("Cannot call a class as a function");
  }
  // Our code:
  this.name = name;
}

new Person('Fred'); // ✅ Okay
Person('George');   // 🔴 Cannot call a class as a function

You might have seen code like this in your bundle. That’s what all those _classCallCheck functions do. (You can reduce the bundle size by opting into the “loose mode” with no checks but this might complicate your eventual transition to real native classes.)

---

By now, you should roughly understand the difference between calling something with new or without new:

	new Person()	Person()
class	✅ this is a Person instance	🔴 TypeError
function	✅ this is a Person instance	😳 this is window or undefined

This is why it’s important for React to call your component correctly. If your component is defined as a class, React needs to use new when calling it.

So can React just check if something is a class or not?

Not so easy! Even if we could tell a class from a function in JavaScript, this still wouldn’t work for classes processed by tools like Babel. To the browser, they’re just plain functions. Tough luck for React.

---

Okay, so maybe React could just use new on every call? Unfortunately, that doesn’t always work either.

With regular functions, calling them with new would give them an object instance as this. It’s desirable for functions written as constructor (like our Person above), but it would be confusing for function components:

function Greeting() {
  // We wouldn’t expect `this` to be any kind of instance here
  return <p>Hello</p>;
}

That could be tolerable though. There are two other reasons that kill this idea.

---

The first reason why always using new wouldn’t work is that for native arrow functions (not the ones compiled by Babel), calling with new throws an error:

const Greeting = () => <p>Hello</p>;
new Greeting(); // 🔴 Greeting is not a constructor

This behavior is intentional and follows from the design of arrow functions. One of the main perks of arrow functions is that they don’t have their own this value — instead, this is resolved from the closest regular function:

class Friends extends React.Component {
  render() {
    const friends = this.props.friends;
    return friends.map(friend =>
      <Friend
        // `this` is resolved from the `render` method
        size={this.props.size}
        name={friend.name}
        key={friend.id}
      />
    );
  }
}

Okay, so arrow functions don’t have their own this. But that means they would be entirely useless as constructors!

const Person = (name) => {
  // 🔴 This wouldn’t make sense!
  this.name = name;
}

Therefore, JavaScript disallows calling an arrow function with new. If you do it, you probably made a mistake anyway, and it’s best to tell you early. This is similar to how JavaScript doesn’t let you call a class without new.

This is nice but it also foils our plan. React can’t just call new on everything because it would break arrow functions! We could try detecting arrow functions specifically by their lack of prototype, and not new just them:

(() => {}).prototype // undefined
(function() {}).prototype // {constructor: f}

But this wouldn’t work for functions compiled with Babel. This might not be a big deal, but there is another reason that makes this approach a dead end.

---

Another reason we can’t always use new is that it would preclude React from supporting components that return strings or other primitive types.

function Greeting() {
  return 'Hello';
}

Greeting(); // ✅ 'Hello'
new Greeting(); // 😳 Greeting {}

This, again, has to do with the quirks of the new operator design. As we saw earlier, new tells the JavaScript engine to create an object, make that object this inside the function, and later give us that object as a result of new.

However, JavaScript also allows a function called with new to override the return value of new by returning some other object. Presumably, this was considered useful for patterns like pooling where we want to reuse instances:

// Created lazily
var zeroVector = null;

function Vector(x, y) {
  if (x === 0 && y === 0) {
    if (zeroVector !== null) {
      // Reuse the same instance
      return zeroVector;
    }
    zeroVector = this;
  }
  this.x = x;
  this.y = y;
}

var a = new Vector(1, 1);
var b = new Vector(0, 0);
var c = new Vector(0, 0); // 😲 b === c

However, new also completely ignores a function’s return value if it’s not an object. If you return a string or a number, it’s like there was no return at all.

function Answer() {
  return 42;
}

Answer(); // ✅ 42
new Answer(); // 😳 Answer {}

There is just no way to read a primitive return value (like a number or a string) from a function when calling it with new. So if React always used new, it would be unable to add support components that return strings!

That’s unacceptable so we need to compromise.

---

What did we learn so far? React needs to call classes (including Babel output) with new but it needs to call regular functions or arrow functions (including Babel output) without new. And there is no reliable way to distinguish them.

If we can’t solve a general problem, can we solve a more specific one?

When you define a component as a class, you’ll likely want to extend React.Component for built-in methods like this.setState(). Rather than try to detect all classes, can we detect only React.Component descendants?

Spoiler: this is exactly what React does.

---

Perhaps, the idiomatic way to check if Greeting is a React component class is by testing if Greeting.prototype instanceof React.Component:

class A {}
class B extends A {}

console.log(B.prototype instanceof A); // true

I know what you’re thinking. What just happened here?! To answer this, we need to understand JavaScript prototypes.

You might be familiar with the “prototype chain”. Every object in JavaScript might have a “prototype”. When we write fred.sayHi() but fred object has no sayHi property, we look for sayHi property on fred’s prototype. If we don’t find it there, we look at the next prototype in the chain — fred’s prototype’s prototype. And so on.

Confusingly, the prototype property of a class or a function does not point to the prototype of that value. I’m not kidding.

function Person() {}

console.log(Person.prototype); // 🤪 Not Person's prototype
console.log(Person.__proto__); // 😳 Person's prototype

So the “prototype chain” is more like __proto__.__proto__.__proto__ than prototype.prototype.prototype. This took me years to get.

What’s the prototype property on a function or a class, then? It’s the __proto__ given to all objects newed with that class or a function!

function Person(name) {
  this.name = name;
}
Person.prototype.sayHi = function() {
  alert('Hi, I am ' + this.name);
}

var fred = new Person('Fred'); // Sets `fred.__proto__` to `Person.prototype`

And that __proto__ chain is how JavaScript looks up properties:

fred.sayHi();
// 1. Does fred have a sayHi property? No.
// 2. Does fred.__proto__ have a sayHi property? Yes. Call it!

fred.toString();
// 1. Does fred have a toString property? No.
// 2. Does fred.__proto__ have a toString property? No.
// 3. Does fred.__proto__.__proto__ have a toString property? Yes. Call it!

In practice, you should almost never need to touch __proto__ from the code directly unless you’re debugging something related to the prototype chain. If you want to make stuff available on fred.__proto__, you’re supposed to put it on Person.prototype. At least that’s how it was originally designed.

The __proto__ property wasn’t even supposed to be exposed by browsers at first because the prototype chain was considered an internal concept. But some browsers added __proto__ and eventually it was begrudgingly standardized (but deprecated in favor of Object.getPrototypeOf()).

And yet I still find it very confusing that a property called prototype does not give you a value’s prototype (for example, fred.prototype is undefined because fred is not a function). Personally, I think this is the biggest reason even experienced developers tend to misunderstand JavaScript prototypes.

---

This is a long post, eh? I’d say we’re 80% there. Hang on.

We know that when we say obj.foo, JavaScript actually looks for foo in obj, obj.__proto__, obj.__proto__.__proto__, and so on.

With classes, you’re not exposed directly to this mechanism, but extends also works on top of the good old prototype chain. That’s how our React class instance gets access to methods like setState:

class Greeting extends React.Component {
  render() {
    return <p>Hello</p>;
  }
}

let c = new Greeting();
console.log(c.__proto__); // Greeting.prototype
console.log(c.__proto__.__proto__); // React.Component.prototype
console.log(c.__proto__.__proto__.__proto__); // Object.prototype

c.render();      // Found on c.__proto__ (Greeting.prototype)
c.setState();    // Found on c.__proto__.__proto__ (React.Component.prototype)
c.toString();    // Found on c.__proto__.__proto__.__proto__ (Object.prototype)

In other words, when you use classes, an instance’s __proto__ chain “mirrors” the class hierarchy:

// `extends` chain
Greeting
  → React.Component
    → Object (implicitly)

// `__proto__` chain
new Greeting()
  → Greeting.prototype
    → React.Component.prototype
      → Object.prototype

2 Chainz.

---

Since the __proto__ chain mirrors the class hierarchy, we can check whether a Greeting extends React.Component by starting with Greeting.prototype, and then following down its __proto__ chain:

// `__proto__` chain
new Greeting()
  → Greeting.prototype // 🕵️ We start here
    → React.Component.prototype // ✅ Found it!
      → Object.prototype

Conveniently, x instanceof Y does exactly this kind of search. It follows the x.__proto__ chain looking for Y.prototype there.

Normally, it’s used to determine whether something is an instance of a class:

let greeting = new Greeting();

console.log(greeting instanceof Greeting); // true
// greeting (🕵️‍ We start here)
//   .__proto__ → Greeting.prototype (✅ Found it!)
//     .__proto__ → React.Component.prototype 
//       .__proto__ → Object.prototype

console.log(greeting instanceof React.Component); // true
// greeting (🕵️‍ We start here)
//   .__proto__ → Greeting.prototype
//     .__proto__ → React.Component.prototype (✅ Found it!)
//       .__proto__ → Object.prototype

console.log(greeting instanceof Object); // true
// greeting (🕵️‍ We start here)
//   .__proto__ → Greeting.prototype
//     .__proto__ → React.Component.prototype
//       .__proto__ → Object.prototype (✅ Found it!)

console.log(greeting instanceof Banana); // false
// greeting (🕵️‍ We start here)
//   .__proto__ → Greeting.prototype
//     .__proto__ → React.Component.prototype 
//       .__proto__ → Object.prototype (🙅‍ Did not find it!)

But it would work just as fine to determine if a class extends another class:

console.log(Greeting.prototype instanceof React.Component);
// greeting
//   .__proto__ → Greeting.prototype (🕵️‍ We start here)
//     .__proto__ → React.Component.prototype (✅ Found it!)
//       .__proto__ → Object.prototype

And that check is how we could determine if something is a React component class or a regular function.

---

That’s not what React does though. 😳

One caveat to the instanceof solution is that it doesn’t work when there are multiple copies of React on the page, and the component we’re checking inherits from another React copy’s React.Component. Mixing multiple copies of React in a single project is bad for several reasons but historically we’ve tried to avoid issues when possible. (With Hooks, we might need to force deduplication though.)

One other possible heuristic could be to check for presence of a render method on the prototype. However, at the time it wasn’t clear how the component API would evolve. Every check has a cost so we wouldn’t want to add more than one. This would also not work if render was defined as an instance method, such as with the class property syntax.

So instead, React added a special flag to the base component. React checks for the presence of that flag, and that’s how it knows whether something is a React component class or not.

Originally the flag was on the base React.Component class itself:

// Inside React
class Component {}
Component.isReactClass = {};

// We can check it like this
class Greeting extends Component {}
console.log(Greeting.isReactClass); // ✅ Yes

However, some class implementations we wanted to target did not copy static properties (or set the non-standard __proto__), so the flag was getting lost.

This is why React moved this flag to React.Component.prototype:

// Inside React
class Component {}
Component.prototype.isReactComponent = {};

// We can check it like this
class Greeting extends Component {}
console.log(Greeting.prototype.isReactComponent); // ✅ Yes

And this is literally all there is to it.

You might be wondering why it’s an object and not just a boolean. It doesn’t matter much in practice but early versions of Jest (before Jest was Good™️) had automocking turned on by default. The generated mocks omitted primitive properties, breaking the check. Thanks, Jest.

The isReactComponent check is used in React to this day.

If you don’t extend React.Component, React won’t find isReactComponent on the prototype, and won’t treat component as a class. Now you know why the most upvoted answer for Cannot call a class as a function error is to add extends React.Component. Finally, a warning was added that warns when prototype.render exists but prototype.isReactComponent doesn’t.

---

You might say this story is a bit of a bait-and-switch. The actual solution is really simple, but I went on a huge tangent to explain why React ended up with this solution, and what the alternatives were.

In my experience, that’s often the case with library APIs. For an API to be simple to use, you often need to consider the language semantics (possibly, for several languages, including future directions), runtime performance, ergonomics with and without compile-time steps, the state of the ecosystem and packaging solutions, early warnings, and many other things. The end result might not always be the most elegant, but it must be practical.

If the final API is successful, its users never have to think about this process. Instead they can focus on creating apps.

But if you’re also curious... it’s nice to know how it works.