How to check if a string is a number

Short answer

Convert the string with Number() and check the result with Number.isFinite(). An empty string needs a separate check, because Number('') is 0:

const isNumeric = (value) => value.trim() !== '' && Number.isFinite(Number(value));
 
console.log(isNumeric('42'));
console.log(isNumeric('-3.5'));
console.log(isNumeric('12px'));
console.log(isNumeric(''));

Output:

true
true
false
false

Only digits

For a quantity, a PIN code or an id, check that the string consists of digits only:

const isDigits = (value) => /^\d+$/.test(value);
 
console.log(isDigits('0042'));
console.log(isDigits('4.2'));

Output:

true
false

A range

After checking the format, check the limits:

const isValidAge = (value) => {
  const age = Number(value);
  return Number.isInteger(age) && age >= 0 && age <= 130;
};
 
console.log(isValidAge('35'), isValidAge('200'), isValidAge('3.5'));

Output:

true false false

Common mistake

parseFloat() reads the number from the beginning and ignores the rest, so it accepts strings that are not numbers:

console.log(parseFloat('12px'), Number('12px'));

Output:

12 NaN

Thanks for reading!

Short answer

Only digits

A range

Common mistake

Functions used in this recipe

Thanks for reading!