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To set a conditional initial value for in React useState:

1. Pass a function to useStatethe hook.
2. Use conditions to determine the correct initial values ​​for state variables.
3. This function will only be called on the initial render.

import {useState} from 'react';

export default function App() {
  // 👇️ passing function to useState
  const [num, setNum] = useState(() => {
    if (2 * 2 === 4) {
      return 4;
    }

    return 42;
  });

  // 👇️ using a ternary
  const [str, setStr] = useState('hi'.length === 2 ? 'hello world' : 'test');

  return (
    <div>
      <h2>num is: {num}</h2>

      <h2>str is: {str}</h2>
    </div>
  );
}

In the first example, we pass a function to useStatethe hook.

During the first render, numthe variable will store useStatewhatever we return from the callback function passed to .

We can use conditionals in functions to determine the correct value for a state variable.

useStatePassing a function to the method is useful when the initial state is the result of an expensive computation .

The function we pass to useStatewill only be called on the initial render.

const [state, setState] = useState(() => {
  const initialState = someExpensiveComputation(props);
  return initialState;
});

This mode is called lazy initialization.

If we need a quick one-line conditional to determine the initial state value, we can use the ternary operator.

import {useState} from 'react';

export default function App() {
  // 👇️ using a ternary
  const [str, setStr] = useState('hi'.length === 2 ? 'hello world' : 'test');

  return (
    <div>
      <h2>str is: {str}</h2>
    </div>
  );
}

The ternary operator is if/elsevery similar to the statement.

If the value to the left of the question mark is true, the operator returns the value to the left of the colon, otherwise it returns the value to the right of the colon.

const result1 = 5 === 5 ? 'yes' : 'no';
console.log(result1); // 👉️ "yes"

const result2 = 5 === 10 ? 'yes' : 'no';
console.log(result2); // 👉️ "no"

If the length of the string hi is 2 characters, we will return hello world as the initial state value, otherwise we return test .