Specifying Dependencies Of A useEffect Hook

The useEffect hook is all about performing side-effects. For instance, you'll want to place API calls within useEffect hooks.

The dependency array -- the second argument to useEffect -- is where you declare all of the values that are depended on within the useEffect. If you're making an API call, this array is likely made up of parameters passed to that call.

Here is a contrived example of what that could look like:

const apiCall = (opts) => Promise.resolve(opts);
const [param1, param2, param3] = [1,2,3];

useEffect(() => {
  const handleApiCall = async () => {
    apiCall({ param1, param2, param3 })
      .then((data) => {
        // do something with the data
      })
      .catch((error) => {
        // do something with the error
      });
  }

  handleApiCall();
}, [param1, param2, param3]);

If you don't specify all of the values used in the body of the useEffect, you are opening yourself up to potentially incorrect code. It is safer to specify all of them. The exhaustive-deps rule can help.

This section of Dan Abramov's "A Complete Guide to useEffect" does an excellent job of showing how things can go wrong when you lie to React about your dependencies.