Javascript – How to call loading function with React useEffect only once

javascriptreact-hooksreactjs

The useEffect React hook will run the passed in function on every change. This can be optimized to let it call only when the desired properties change.

What if I want to call an initialization function from componentDidMount and not call it again on changes? Let's say I want to load an entity, but the loading function doesn't need any data from the component. How can we make this using the useEffect hook?

class MyComponent extends React.PureComponent {
    componentDidMount() {
        loadDataOnlyOnce();
    }
    render() { ... }
}

With hooks this could look like this:

function MyComponent() {
    useEffect(() => {
        loadDataOnlyOnce(); // this will fire on every change :(
    }, [...???]);
    return (...);
}

Best Answer

If you only want to run the function given to useEffect after the initial render, you can give it an empty array as second argument.

function MyComponent() {
  useEffect(() => {
    loadDataOnlyOnce();
  }, []);

  return <div> {/* ... */} </div>;
}
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