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.

Copied!
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.

Copied!
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.

Copied!
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.

Copied!
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.

Copied!
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.

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

Copied!
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.

Copied!
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.