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.

You will learn

  • What purity is and how it helps you avoid bugs
  • How to keep components pure by keeping changes out of the render phase
  • How to use Strict Mode to find mistakes in your components

Purity: Components as formulas

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

  • 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.

You might already be familiar with one example of pure functions: formulas in math.

Consider this math formula: y = 2x.

If x = 2 then y = 4. Always.

If x = 3 then y = 6. Always.

If x = 3, y won’t sometimes be 9 or –1 or 2.5 depending on the time of day or the state of the stock market.

If y = 2x and x = 3, y will always be 6.

If we made this into a JavaScript function, it would look like this:

function double(number) {
  return 2 * number;

In the above example, double() is a pure function. If you pass it 3, it will return 6. Always.

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:

function Recipe({ drinkers }) {
  return (
      <li>Boil {drinkers} cups of milk.</li>
      <li>Add {2 * drinkers} spoons of masala spices.</li>
      <li>Remove from heat, and add {drinkers} spoons of tea.</li>

export default function App() {
  return (
      <h1>Spiced Chai Recipe</h1>
      <h2>For one</h2> 
      <Recipe drinkers={1} />
      <h2>For a gathering</h2>
      <Recipe drinkers={4} />

When you pass drinkers={1} to Recipe, it will return JSX containing 1 cups of milk. Always.

If you pass drinkers={4}, it will return JSX containing 4 cups of milk. Always.

Just like a math formula.

You could think of your components as recipes: if you follow them and don’t introduce new ingredients during the cooking process, you will get the same dish every time. That “dish” is the JSX that the component serves to React to render.

A tea recipe for x people: take x cups of water, add 2x spoons of spices, and x spoons of tea!

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.

Going back to our formula y = 2x, now even if x = 2, we cannot trust that y = 4. Our tests could fail, our users would be baffled, planes would fall out of the sky—you can see how this would lead to confusing bugs!

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. It doesn’t matter if you call y = 2x before or after y = 5x: both formulas will resolve independently of each other. In the same way, each component should only “think for itself,” and not attempt to coordinate with or depend upon others during rendering. Rendering is like a school exam: each component should calculate JSX on their own!

Deep Dive

Detecting impure calculations with StrictMode

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. You’ll be surprised how far this can take you!

Deep Dive

Why does React care about purity?


  • A component must be pure, meaning:
    • Mind its own business. It should not change any objects or variables that existed before rendering.
    • Same inputs, same output. Given the same inputs, a component should always return the same JSX.
  • Rendering can happen at any time, so components should not depend on each others’ rendering sequence.
  • You should not mutate any of the inputs that your components use for rendering. That includes props, state, and context. To update the screen, “set” state instead of mutating preexisting objects.
  • Strive to express your component’s logic in the JSX you return. When you need to “change things,” you’ll usually want to do it in an event handler. As a last resort, you can useEffect.
  • Writing pure functions takes a bit of practice, but it unlocks the power of React’s paradigm.

Challenge 1 of 3:
Fix a broken clock

This component tries to set the <h1>’s CSS class to "night" during the time from midnight to six hours in the morning, and "day" at all other times. However, it doesn’t work. Can you fix this component?

You can verify whether your solution works by temporarily changing the computer’s timezone. When the current time is between midnight and six in the morning, the clock should have inverted colors!

export default function Clock({ time }) {
  let hours = time.getHours();
  if (hours >= 0 && hours <= 6) {
    document.getElementById('time').className = 'night';
  } else {
    document.getElementById('time').className = 'day';
  return (
    <h1 id="time">