useEffect runs infinite loop despite no change in dependencies

Question

I have Function Component that utilizes hooks. In the useEffect hook, I simply want to fetch data from my back end and store the results in state. However, despite adding the data variable as a dependency, useEffect still fires on an infinite loop - even though the data hasn't changed. How can I stop useEffect from firing continuously?

I've tried the empty array hack, which DOES stop useEffect from continuously firing, but it's not the desired behavior. If the user saves new data, for example, useEffect should fire again to get updated data - I'm not looking to emulate componentDidMount.

const Invoices = () => {
  const [invoiceData, setInvoiceData] = useState([]);

  useEffect(() => {
    const updateInvoiceData = async () => {
      const results = await api.invoice.findData();
      setInvoiceData(results);
    };
    updateInvoiceData();
  }, [invoiceData]);

  return (
    <Table entries={invoiceData} />
  );
};

I expected useEffect to fire after the initial render, and again ONLY when invoiceData changes.

Have a look at my explanation here stackoverflow.com/questions/57847626/… — Milind Agrawal
Isn't this the same behavior as an empty dependency array? if (!invoiceData) updateInvoiceData(); — Trevor
It will keep on getting called if there is some value in invoiceData. The second argument in useEffect will handle that better and will only execute the code when value in invoiceData is changed. But if the value is getting changed every time you are making a call then this won't work. — Milind Agrawal
I would suggest the move the updateInvoiceData logic to separate method and useuseEffect only for the first time and then whenever new data is saved, call the method again(Do not use useEffect) — Milind Agrawal

jered jered · Accepted Answer · 2019-09-09T18:40:43

The way the useEffect dependency array works is by checking for strict (===) equivalency between all of the items in the array from the previous render and the new render. Therefore, putting an array into your useEffect dependency array is extremely hairy because array comparison with === checks equivalency by reference not by contents.

const foo = [1, 2, 3];
const bar = foo;
foo === bar; // true

const foo = [1, 2, 3];
const bar = [1, 2, 3];
foo === bar; // false

Inside of your effect function, when you do setInvoiceData(results) you are updating invoiceData to a new array. Even if all the items inside of that new array are exactly the same, the reference to the new invoiceData array has changed, causing the dependencies of the effect to differ, triggering the function again -- ad infinitum.

One simple solution is to simply remove invoiceData from the dependency array. In this way, the useEffect function basically acts similar to componentDidMount in that it will trigger once and only once when the component first renders.

useEffect(() => {
    const updateInvoiceData = async () => {
      const results = await api.invoice.findData();
      setInvoiceData(results);
    };
    updateInvoiceData();
  }, []);

This pattern is so common (and useful) that it is even mentioned in the official React Hooks API documentation:

If you want to run an effect and clean it up only once (on mount and unmount), you can pass an empty array ([]) as a second argument. This tells React that your effect doesn’t depend on any values from props or state, so it never needs to re-run. This isn’t handled as a special case — it follows directly from how the dependencies array always works.

useEffect runs infinite loop despite no change in dependencies

3 Answers