Hooks tip! Avoid infinite recursion in React.useEffect()

React 16.8.0 with Hooks was released today. A big deal. Executive summary; components as functions is all the rage now.

What used to be this:

class MyComponent extends React.Component {
  ...

  componentDidMount() {
    ...
  }
  componentDidUpdate() {
    ...
  }

  render() { STUFF }
}

...is now this:

function MyComponent() {
  ...

  React.useEffect(() => {
    ...
  })

  return STUFF
}

Inside the useEffect "side-effect callback" you can actually update state. But if you do, and this is no different that old React.Component.componentDidUpdate, it will re-run the side-effect callback. Here's a simple way to cause an infinite recursion:

// DON'T DO THIS

function MyComponent() {
  const [counter, setCounter] = React.useState(0);

  React.useEffect(() => {
    setCounter(counter + 1);
  })

  return <p>Forever!</p>
}

The trick is to pass a second argument to React.useEffect that is a list of states to exclusively run on.

Here's how to fix the example above:

function MyComponent() {
  const [counter, setCounter] = React.useState(0);
  const [times, setTimes] = React.useState(0);

  React.useEffect(
    () => {
      if (times % 3 === 0) {
        setCounter(counter + 1);
      }
    },
    [times]  // <--- THIS RIGHT HERE IS THE KEY!
  );

  return (
    <div>
      <p>
        Divisible by 3: {counter}
        <br />
        Times: {times}
      </p>
      <button type="button" onClick={e => setTimes(times + 1)}>
        +1
      </button>
    </div>
  );
}

You can see it in this demo.

Note, this isn't just about avoiding infinite recursion. It can also be used to fit your business logic and/or an optimization to avoid executing the effect too often.