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When we use useStatethe hook to declare an empty state array but do not type the array, the error "Type is not assignable to type 'never'" appears. To resolve the error, use generics to type the state array, for example const [arr, setArr] = useState<string[]>([]).

Below is sample code that produces the above error.

import {useState} from 'react';

function App() {
  // 👇️ arr is never[]
  const [arr, setArr] = useState([]);

  // ⛔️ Error: Type 'number' is not assignable to type 'never'.ts(2322)
  setArr([1, 2, 3]);

  return (
    <div className="App">
      <div>Hello world</div>
    </div>
  );
}

export default App;

The error is because we declared an empty states array without explicitly typing it.

The type of the array is inferred to be never[], in other words, an array that is always empty, which is not what we want.

To fix the error, useStateuse generics on the hook to type the state array.

import {useState} from 'react';

function App() {
  // 👇️ type the array with the generic
  const [arr, setArr] = useState<any[]>([]);

  setArr([1, 2, 3]);

  return (
    <div className="App">
      <div>Hello world</div>
    </div>
  );
}

export default App;

We use a very general any[]type, which is an array containing elements of any type.

Where possible, be as specific as possible.

import {useState} from 'react';

function App() {
  // 👇️ array of strings
  const [strArr, setStrArr] = useState<string[]>([]);

  // 👇️ an array of objects
  const [objArr, setObjArr] = useState<{name: string; age: number}[]>([]);

  setStrArr(['a', 'b', 'c']);

  setObjArr([{name: 'A', age: 1}]);

  return (
    <div className="App">
      <div>Hello world</div>
    </div>
  );
}

export default App;

The examples above show how to type the state array as either an array of strings or an array of objects.

TypeScriptAlways make sure to explicitly type empty arrays in React when using .

If we get neveran error message containing a type, there is a good chance that you forgot to explicitly type a value and it was inferred to be of nevertype .

To debug this, hover over the value to see its type and look for ways to explicitly type it. If we were using React hooks, there’s a good chance you’d have to use generics like we did with the useState hook.

If we were entering a basic TypeScript variable, we would simply separate the variable name and its type with a colon.

function App() {
  // 👇️ declare array of strings
  const arr: string[] = [];

  arr.push('a', 'b', 'c');

  return (
    <div className="App">
      <div>Hello world</div>
    </div>
  );
}

export default App;

Summarize

When we use useStatethe hook to declare an empty state array but do not type the array, the error "Type is not assignable to type 'never'" appears. To resolve the error, use generics to type the state array, for example const [arr, setArr] = useState<string[]>([]).