Why can't React Hooks be called inside loops or nested function?

Question

React Hooks documentation says to not call Hooks inside loops, conditions, or nested functions.

I understood that the order of execution was important so React can know which state corresponds to which useState call. Given that, it's obvious that a hook cannot be called inside a condition.

But I don't see what is the problem if we call useState inside a loop where the number of iterations doen't change over the time. Here is an example :

const App = () => {   const inputs = [];    for(let i = 0; i < 10; i++) {     inputs[i] = useState('name' + i);   }    return inputs.map(([value, setValue], index) => (     <div key={index}>        <input value={value} onChange={e => setValue(e.target.value)} />     </div>   )); }  export default App; 

Is there a problem with this code above ? And what is the problem of calling useState inside a nested function, if this function is called on every render ?

Answer 1

The reference states the actual cause,

By following this rule, you ensure that Hooks are called in the same order each time a component renders. That’s what allows React to correctly preserve the state of Hooks between multiple useState and useEffect calls.

and provides the example that shows why this is important.

Loops, conditions and nested functions are common places where the order in which hooks are executed may be disturbed. If a developer is sure that a loop, etc. is justified and guarantees the order then there's no problem.

In fact, a loop would be considered valid custom hook if it was extracted to a function, a linter rule can be disabled where needed (a demo):

// eslint-disable-next-line react-hooks/rules-of-hooks const useInputs = n => [...Array(n)].map((_, i) => useState('name' + i)); 

The example above won't cause problems but a loop isn't necessarily justified; it can be a single array state:

const App = () => {   const [inputs, setInputs] = useState(Array(10).fill(''));   const setInput = (i, v) => {     setInputs(Object.assign([...inputs], { [i]: v }));   };    return inputs.map((v, i) => (     <div key={i}>        <input value={v} onChange={e => setInput(i, e.target.value)} />     </div>   )); }