Keeping Components Pure

Some JavaScript functions are pure. Pure functions only perform a calculation and nothing more. By strictly only writing your components as pure functions, you can avoid an entire class of baffling bugs and unpredictable behavior as your codebase grows. To get these benefits, though, there are a few rules you must follow.

Purity: Components as formulas

In computer science (and especially the world of functional programming), a pure function is a function with the following characteristics:

• It minds its own business. It does not change any objects or variables that existed before it was called.
• Same inputs, same output. Given the same inputs, a pure function should always return the same result.

React is designed around this concept. React assumes that every component you write is a pure function. This means that React components you write must always return the same JSX given the same inputs.

Side Effects: (un)intended consequences

React’s rendering process must always be pure. Components should only return their JSX, and not change any objects or variables that existed before rendering—that would make them impure!

Here is a component that breaks this rule:

let guest = 0;

function Cup() {
  // Bad: changing a preexisting variable!
  guest = guest + 1;
  return <h2>Tea cup for guest #{guest}</h2>;
}

export default function TeaSet() {
  return (
    <>
      <Cup />
      <Cup />
      <Cup />
    </>
  );
}

This component is reading and writing a guest variable declared outside of it. This means that calling this component multiple times will produce different JSX! And what’s more, if other components read guest, they will produce different JSX, too, depending on when they were rendered! That’s not predictable.

You can fix this component by passing guest as a prop instead:

function Cup({ guest }) {
  return <h2>Tea cup for guest #{guest}</h2>;
}

export default function TeaSet() {
  return (
    <>
      <Cup guest={1} />
      <Cup guest={2} />
      <Cup guest={3} />
    </>
  );
}

Now your component is pure, as the JSX it returns only depends on the guest prop. In general, you should not expect your components to be rendered in any particular order.

Detecting impure calculations with StrictMode

In React there are three kinds of inputs that you can read while rendering: props, state, and context. You should always treat these inputs as read-only. When you want to change something in response to user input, you should set state instead of writing to a variable. You should never change preexisting variables or objects while your component is rendering.

React offers a “Strict Mode” in which it calls each component’s function twice during development. By calling the component functions twice, Strict Mode helps find components that break these rules. Strict Mode has no effect in production, so it won’t slow down the app for your users. To opt into Strict Mode, you can wrap your root component into <React.StrictMode>. Some frameworks do this by default.

Local mutation: Your component’s little secret

In the above example, the problem was that the component changed a preexisting variable while rendering. This is often called a “mutation” to make it sound a bit scarier. Pure functions don’t mutate variables outside of the function’s scope or objects that were created before the call—that makes them impure!

However, it’s completely fine to change variables and objects that you’ve just created while rendering. In this example, you create an [] array, assign it to a cups variable, and then push a dozen cups into it:

function Cup({ guest }) {
  return <h2>Tea cup for guest #{guest}</h2>;
}

export default function TeaGathering() {
  let cups = [];
  for (let i = 1; i <= 12; i++) {
    cups.push(<Cup key={i} guest={i} />);
  }
  return cups;
}

If the cups variable or the [] array were created outside the TeaGathering function, this would be a huge problem! You would be changing a preexisting object by pushing items into that array.

However, it’s fine because you’ve created them during the same render, inside TeaGathering. No code outside of TeaGathering will ever know that this happened. This is called “local mutation”—it’s like your component’s little secret.

Where you can cause side effects

While functional programming relies heavily on purity, at some point, somewhere, something has to change. That’s kind of the point of programming! These changes—updating the screen, starting an animation, changing the data—are called side effects. They’re things that happen “on the side”, not during rendering.

In React, side effects usually belong inside event handlers. Event handlers are functions that React runs when you perform some action—for example, when you click a button. Even though event handlers are defined inside your component, they don’t run during rendering! So event handlers don’t need to be pure.

If you’ve exhausted all other options and can’t find the right event handler for your side effect, you can still attach it to your returned JSX with a useEffect call in your component. This tells React to execute it later, after rendering, when side effects are allowed. However, this approach should be your last resort. When possible, try to express your logic with rendering alone.

Why does React care about purity?

Writing pure functions takes some habit and discipline. But it also unlocks marvelous opportunities:

• Your components could run in a different environment—for example, on the server! Since they return the same result for the same inputs, one component can serve many user requests.
• You can improve performance by skipping rendering components whose inputs have not changed. This is safe because pure functions always return the same results, so they are safe to cache.
• If some data changes in the middle of rendering a deep component tree, React can restart rendering without wasting time to finish the outdated render. Purity makes it safe to stop calculating at any time.

Every new React feature we’re building takes advantage of purity. From data fetching to animations to performance, keeping components pure unlocks the power of the React paradigm.