Can I add a React input event handler only on certain conditions?

Question

I want to add the onFocus event handler on my element only if a condition is true. Right now, I was hoping to be able to do a ternary on the onFocus itself, so something like this:

<input
  type='text'
  className='some-class'
  value={value}
  onFocus={useTheOnFocus ? evt => onFocusHandler(evt) : null}
  onChange={evt => onChangeHandler(evt.target.value)}
/>

This however, does not work and generates a series of errors and warnings:

Uncaught TypeError: handler.apply is not a function

Warning: Can't call setState (or forceUpdate) on an unmounted component. This is a no-op, but it indicates a memory leak in your application. To fix, cancel all subscriptions and asynchronous tasks in the componentWillUnmount method.

I know the alternative would be to just write the input tag twice within it's own ternary one with the onFocus and the other without it. But I would like to do it in a more condensed matter. Is there another simpler and more condensed way to get this done?

Instead of null just use () => {}?

Patrick Roberts
– Patrick Roberts

2018-07-17 15:09:34 +00:00
Commented Jul 17, 2018 at 15:09 — Patrick Roberts
– Patrick Roberts, Commented Jul 17, 2018 at 15:09
Oops! I had completely forgotten.

theJuls
– theJuls

2018-07-27 13:27:06 +00:00
Commented Jul 27, 2018 at 13:27 — theJuls
– theJuls, Commented Jul 27, 2018 at 13:27

Oleg Yakovlev · Accepted Answer · 2018-07-17 15:19:32Z

2

you can add this

onFocus={useTheOnFocus ? onFocusHandler : undefined}

or set a handler

onFocus={onFocusHandler}

and check a condition in a handler

onFocusHandler = (ev) => {
   if(!condition) {
      return null;
   }
   // your code
}

answered Jul 17, 2018 at 15:19

Oleg Yakovlev

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Comments

Tholle · Accepted Answer · 2018-07-17 15:24:07Z

You can give event handlers a value of undefined if you don't want them to be active.

Example

class App extends React.Component {
  state = { useTheOnFocus: false };

  componentDidMount() {
    setInterval(() => {
      this.setState(prevState => ({
        useTheOnFocus: !prevState.useTheOnFocus
      }));
    }, 2000);
  }

  onFocusHandler = evt => {
    evt.preventDefault();
    console.log("Focus handler active");
  };

  render() {
    const { useTheOnFocus } = this.state;

    return (
      <input
        type="text"
        className="some-class"
        onFocus={useTheOnFocus ? this.onFocusHandler : undefined}
      />
    );
  }
}

ReactDOM.render(<App />, document.getElementById("root"));

<script src="https://cdnjs.cloudflare.com/ajax/libs/react/15.1.0/react.min.js"></script>
<script src="https://cdnjs.cloudflare.com/ajax/libs/react/15.1.0/react-dom.min.js"></script>

<div id="root"></div>

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