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Check if the input value is a valid number in React:

1. Checks that the input value is not an empty string or contains only whitespace.
2. Pass a value to isNaN()the function.
3. If isNaNreturns false , the value is a valid number.

import {useState} from 'react';

function isNumber(str) {
  if (str.trim() === '') {
    return false;
  }

  return !isNaN(str);
}

export default function App() {
  const [num, setNum] = useState('');

  const handleChange = event => {
    setNum(event.target.value);

    if (isNumber(event.target.value)) {
      console.log('✅ It is a valid number');
    } else {
      console.log('⛔️ It is NOT a valid number');
    }
  };

  return (
    <div>
      <input
        id="num"
        name="num"
        type="text"
        onChange={handleChange}
        value={num}
      />
    </div>
  );
}

This example shows how to check whether the value of an input field is a valid number.

We set onChangethe property on the input field, so every time its value changes, handleChangethe function will be called.

In our handleChangefunction, we update numthe state variable and check if the value is a valid number.

请注意, the value of an input field is always of type string, regardless of whether its Type property is set to Text or Number.

In our isNumberfunction, we check that the string passed in is not empty and does not contain only spaces.

We then use isNaNthe function to check if the provided string is not a number.

We use the logical NOT !operator to negate isNaNthe value returned by the function.

isNaN( not a number ) The function attempts to convert a string to a number and returns true if it fails .

console.log(isNaN('hello')); // 👉️ true
console.log(isNaN('')); // 👉️ false
console.log(isNaN('   ')); // 👉️ false

This may seem confusing at first, but isNaNthe function converts an empty string or a string containing spaces to a number and results in the value 0, so it returns false .

console.log(Number('')); // 👉️ 0
console.log(Number('   ')); // 👉️ 0

That's why we use trim()the method to trim the string and verify that it is not an empty string.

We know that if isNaNthe function is called with a string containing at least 1 character and returns true, then the string is not a valid number.

Conversely, if the isNaN function is called with a string containing at least 1 character and returns false, then the string is a valid number.

Alternatively, we can use a regular expression to only allow users to enter numbers in your input field.

If you want to create an input field that only allows numbers, check out my other article - Creating a Number-Only Input Field in React .