Check if a Character is a Number using JavaScript

# Table of Contents

1. Check if a Character in String is a Number with Regex
2. Check if a Character in String is a Number using isNaN()

# Check if a Character in String is a Number with Regex

Use the RegExp.test() method to check if a character in a string is a number.

The test() method will return true if the character is a valid number and false otherwise.

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function isNumber(char) {
  return /^\d$/.test(char);
}

const str = 'a1';

console.log(isNumber(str[0])); // 👉️ false
console.log(isNumber(str[1])); // 👉️ true
console.log(isNumber('100')); // 👉️ false
console.log(isNumber('')); // 👉️ false
console.log(isNumber(undefined)); // false

We used the RegExp.test() method to test a regular expression against the character.

The forward slashes / / mark the start and end of the regular expression.

The caret ^ matches the beginning of the input and the dollar sign $ matches the end of the input.

The \d character matches any digit from 0 to 9.

If the character we pass to the function is a number, the isNumber() function returns true, otherwise, false is returned.

If you also want to return true for multiple characters if they are numbers, e.g. 100, update the regular expression.

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function isNumber(char) {
  return /^\d+$/.test(char);
}

const str = 'a1';

console.log(isNumber(str[0])); // 👉️ false
console.log(isNumber(str[1])); // 👉️ true
console.log(isNumber('100')); // 👉️ true
console.log(isNumber('')); // 👉️ false
console.log(isNumber(undefined)); // false

We added the plus + special character to the regular expression.

The plus + matches the preceding item (a digit) 1 or more times.

In its entirety, the regular expression matches a string that starts with and ends with a digit and contains one or more digits.

Alternatively, you can use the isNan() function.

# Check if a Character in String is a Number using isNaN()

You can also use the isNan() function to check if a character is a number.

The function checks if the provided value is NaN (not a number). If the function returns false, then the character is a valid number.

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function isNumber(char) {
  if (typeof char !== 'string') {
    return false;
  }

  if (char.trim() === '') {
    return false;
  }

  return !isNaN(char);
}

const str = 'a1';
console.log(isNumber(str[0])); // 👉️ false
console.log(isNumber(str[1])); // 👉️ true
console.log(isNumber('123')); // 👉️ true
console.log(isNumber('')); // 👉️ false
console.log(isNumber(undefined)); // false

The isNumber() function returns true if the supplied string is a number and false otherwise.

We first check if the value passed to the function is not of type string, in which case we return false straight away.

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if (typeof char !== 'string') {
  return false;
}

Then we check if the provided value is an empty string or contains only whitespace, in which case we also return false.

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if (char.trim() === '') {
  return false;
}

Finally, we use the isNaN() function to check if the provided character is not a number.

The logical NOT (!) operator is used to negate the value returned from the isNaN (is not a number) function.

The isNaN function tries to convert the string to a number and returns true if it fails.

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

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

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

We know that if the isNaN() function gets called with a string that contains at least 1 character and returns true, then the string is NOT a valid number.

Here are some more examples of calling isNaN() with strings.

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console.log(isNaN('123')); // 👉️ false
console.log(isNaN('1.23')); // 👉️ false
console.log(isNaN('1,23')); // 👉️ true
console.log(isNaN('123test')); // 👉️ true

Which approach you pick is a matter of personal preference. I'd use the RegExp.test() method because I find it more direct and intuitive.