In Typescript, How to check if a string is Numeric

Question

In Typescript, this shows an error saying isNaN accepts only numeric values

isNaN(\'9BX46B6A\')

and this returns false because parseF

Answer 1

Whether a string can be parsed as a number is a runtime concern. Typescript does not support this use case as it is focused on compile time (not runtime) safety.

Answer 2

Most of the time the value that we want to check is string or number, so here is function that i use:

const isNumber = (n: string | number): boolean => 
    !isNaN(parseFloat(String(n))) && isFinite(Number(n));

---

Codesandbox tests.

const willBeTrue = [0.1, '1', '-1', 1, -1, 0, -0, '0', "-0", 2e2, 1e23, 1.1, -0.1, '0.1', '2e2', '1e23', '-0.1', ' 898', '080']

const willBeFalse = ['9BX46B6A', "+''", '', '-0,1', [], '123a', 'a', 'NaN', 1e10000, undefined, null, NaN, Infinity, () => {}]

Answer 3

Simple answer: (watch for blank & null)

isNaN(+'111') = false;
isNaN(+'111r') = true;
isNaN(+'r') = true;
isNaN(+'') = false;   
isNaN(null) = false;   

https://codepen.io/CQCoder/pen/zYGEjxd?editors=1111

Answer 4

I would choose an existing and already tested solution. For example this from rxjs in typescript:

function isNumeric(val: any): val is number | string {
  // parseFloat NaNs numeric-cast false positives (null|true|false|"")
  // ...but misinterprets leading-number strings, particularly hex literals ("0x...")
  // subtraction forces infinities to NaN
  // adding 1 corrects loss of precision from parseFloat (#15100)
  return !isArray(val) && (val - parseFloat(val) + 1) >= 0;
}

rxjs/isNumeric.ts

Without rxjs isArray() function and with simplefied typings:

function isNumeric(val: any): boolean {
  return !(val instanceof Array) && (val - parseFloat(val) + 1) >= 0;
}

You should always test such functions with your use cases. If you have special value types, this function may not be your solution. You can test the function here.

Results are:

enum         : CardTypes.Debit   : true
decimal      : 10                : true
hexaDecimal  : 0xf10b            : true
binary       : 0b110100          : true
octal        : 0o410             : true
stringNumber : '10'              : true

string       : 'Hello'           : false
undefined    : undefined         : false
null         : null              : false
function     : () => {}          : false
array        : [80, 85, 75]      : false
turple       : ['Kunal', 2018]   : false
object       : {}                : false

As you can see, you have to be careful, if you use this function with enums.

Answer 5

You can use the Number.isFinite() function:

Number.isFinite(Infinity);  // false
Number.isFinite(NaN);       // false
Number.isFinite(-Infinity); // false
Number.isFinite('0');       // false
Number.isFinite(null);      // false

Number.isFinite(0);         // true
Number.isFinite(2e64);      // true

Note: there's a significant difference between the global function isFinite() and the latter Number.isFinite(). In the case of the former, string coercion is performed - so isFinite('0') === true whilst Number.isFinite('0') === false.

Also, note that this is not available in IE!

Answer 6

The way to convert a string to a number is with Number, not parseFloat.

Number('1234') // 1234
Number('9BX9') // NaN

You can also use the unary plus operator if you like shorthand:

+'1234' // 1234
+'9BX9' // NaN

Be careful when checking against NaN (the operator === and !== don't work as expected with NaN). Use:

 isNaN(+maybeNumber) // returns true if NaN, otherwise false