How to Reliably Check if a Javascript String is a Number

Question

How do you determine if a string represents a number?

The straightforward and potentially naive way is this:

function is_number(my_str) {
  return !isNaN(parseInt(my_str));
}

However, the above does not work. Notice that the following all return true:

is_number("3, 4, 5");
is_number("3, (123)");
is_number("3, 123)");
is_number("3, (123) (and foo bar)");
is_number("3 apples and 2 oranges");

The issue is that parseInt seems to only look at the first word. Any idea on a more comprehensive solution here?

Maheer Ali · Accepted Answer · 2019-02-04 20:14:44Z

1

You can use Number(). parseInt and parseFloat converts String to Number if first character is a number. But Number() checks for whole String.Number return NaN if whole string is not number

function is_number(my_str) {
  return !isNaN(Number(my_str)) && Boolean(my_str) || my_str == 0;
}

console.log(is_number("3, 4, 5"));
console.log(is_number("3, (123)"));
console.log(is_number("3, 123)"));
console.log(is_number("3, (123) (and foo bar)"));
console.log(is_number("3 apples and 2 oranges"));
console.log(is_number(null));
console.log(is_number(undefined));
console.log(is_number(0));
console.log(is_number('0'));

edited Feb 4, 2019 at 20:14

answered Feb 4, 2019 at 19:50

Maheer Ali

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3 Comments

speedplane Over a year ago

Getting close, but notice is_number(null) == true

speedplane Over a year ago

I know I'm expanding the scope of this function slightly, but notice: is_number(0) == false

Maheer Ali Over a year ago

@speedplane ok now check.

Code Maniac · Accepted Answer · 2019-02-04 20:02:41Z

0

All you need is Number

console.log(Number("3, 4, 5") || false)
console.log(Number("3, (123)") || false)
console.log(Number("3, 123)") || false)
console.log(Number('123') || false)

edited Feb 4, 2019 at 20:02

answered Feb 4, 2019 at 19:53

Code Maniac

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Comments

Phil Booth · Accepted Answer · 2019-02-05 11:17:14Z

0

Here's my regular-expression based effort:

function isNumber (string) {
  return /^-?[0-9]+(?:\.[0-9]+)?(?:[eE][-+]?[0-9]+)?$/.test(string);
}

Includes support for leading sign, decimal places and exponent.

If it returns true, the string can be converted to a number using JSON.parse:

function isNumber (string) {
  return /^-?[0-9]+(?:\.[0-9]+)?(?:[eE][-+]?[0-9]+)?$/.test(string);
}

function convertNumber (string) {
  if (isNumber(string)) {
    return JSON.parse(string);
  }
}

[
  '0',
  '1.2',
  '-3.4',
  '5.6789e100',
  '0.1e-100',
  '0.1e+100',
].forEach(string => {
  console.log(`isNumber('${string}'):`, isNumber(string));
  console.log(`typeof convertNumber('${string}'):`, typeof convertNumber(string));
});

edited Feb 5, 2019 at 11:17

answered Feb 4, 2019 at 20:18

Phil Booth

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