React: setState for primitive value doesn't work with ===-CodePudding

I have a variable that stores the index of a selected item. I used a conditional expression based on that variable to add/remove a class name so said item is rendered in a distinguishing way. I boiled down the problem to this snippet:

function App() {
  const [selectedItem, setSelectedItem] = setState(-1);

  setSelectedItem(0);
  console.log(selectedItem);

  return (
    <>
    {selectedItem !== 0 && <p>no item selected</p>}
    {selectedItem === 0 && <p>item {selectedItem} selected</p>}
    </>
  );
}

This snippet always displays that no item is selected.

CodePudding user response：

The hook is called useState, not setState.

Calling setSelectedItem inside the render function will trigger an infinite loop. You need to move it to an effect hook so it only runs one.

export default function App() {
  const [selectedItem, setSelectedItem] = useState(-1);

  useEffect(() => {
    setSelectedItem(0);
  }, []);
  
  console.log(selectedItem);

  return (
    <>
    {selectedItem !== 0 && <p>no item selected</p>}
    {selectedItem === 0 && <p>item {selectedItem} selected</p>}
    </>
  );
}

CodePudding user response：

What is setState ? Do you mean useState ? And also, you shouldn't update the state like this, you should do it in a useEffect, and use an empty array as dependency, so it will run only once, when your component is being mounted:

useEffect(() => {
    setSelectedItem(0);
},[])

Then it is working like a charm

CodePudding user response：

Replace === and !== with == and !=, respectively, and voila, it works. Alas, a warning is reported that one shall use === instead.

selectedItem is an array, apparently primitives make bad references. Still, it is bizarre to some extent that inside <p> the variable is unboxed automatically, while for evaluating === it isn't and thus an array is compared to a primitive, which is not true, no matter the values. == compares every shit in JS that you feed it with, so here it works.

Hope, this saves somebody 2 hours of debugging.

If somebody has a correct workaround for this, please share below.