Check if a Character is a Number using JavaScript

Check if Character in String is a Number #

To check if a character is a number, pass the character as a parameter to the isNaN() function. 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

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

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

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

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

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

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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 use 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.

Conversely, if the isNaN function gets called with a string that contains at least 1 character and returns false, then the string is 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

An alternative solution is to use a regular expression.

Check if Character in String is a Number with Regex #

Use the test() method on the following regular expression to check if a character is a number - /^\d$/. The test method will return true if the character is a valid number, otherwise it will return false.

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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 created a reusable function, which checks if the provided character is a number.

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 $ - the end of the input.

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

If you ever need help reading a regular expression, check out this regex cheatsheet from MDN. It's by far the best one out there.

If the character we pass as a parameter to the function is a number, the isNumber function will return true, in all other cases it returns false.

Further Reading #

• How to check if a Character is a Letter in JavaScript
• Capitalize the First Letter of Each Word in Array in JS