(Built-in) way in JavaScript to check if a string

Question

I'm hoping there's something in the same conceptual space as the old VB6 IsNumeric() function?

Answer 1

You could make use of types, like with the flow library, to get static, compile time checking. Of course not terribly useful for user input.

// @flow

function acceptsNumber(value: number) {
  // ...
}

acceptsNumber(42);       // Works!
acceptsNumber(3.14);     // Works!
acceptsNumber(NaN);      // Works!
acceptsNumber(Infinity); // Works!
acceptsNumber("foo");    // Error!

Answer 2

If you're just trying to check if a string is a whole number (no decimal places), regex is a good way to go. Other methods such as isNaN are too complicated for something so simple.

function isNumeric(value) {
    return /^-{0,1}\d+$/.test(value);
}

console.log(isNumeric('abcd'));         // false
console.log(isNumeric('123a'));         // false
console.log(isNumeric('1'));            // true
console.log(isNumeric('1234567890'));   // true
console.log(isNumeric('-23'));          // true
console.log(isNumeric(1234));           // true
console.log(isNumeric('123.4'));        // false
console.log(isNumeric(''));             // false
console.log(isNumeric(undefined));      // false
console.log(isNumeric(null));           // false

To only allow positive whole numbers use this:

function isNumeric(value) {
    return /^\d+$/.test(value);
}

console.log(isNumeric('123'));          // true
console.log(isNumeric('-23'));          // false

Answer 3

I have tested and Michael's solution is best. Vote for his answer above (search this page for "If you really want to make sure that a string" to find it). In essence, his answer is this:

function isNumeric(num){
  num = "" + num; //coerce num to be a string
  return !isNaN(num) && !isNaN(parseFloat(num));
}

It works for every test case, which I documented here: https://jsfiddle.net/wggehvp9/5/

Many of the other solutions fail for these edge cases: ' ', null, "", true, and []. In theory, you could use them, with proper error handling, for example:

return !isNaN(num);

or

return (+num === +num);

with special handling for /\s/, null, "", true, false, [] (and others?)

Answer 4

You can use the result of Number when passing an argument to its constructor.

If the argument (a string) cannot be converted into a number, it returns NaN, so you can determinate if the string provided was a valid number or not.

Notes: Note when passing empty string or '\t\t' and '\n\t' as Number will return 0; Passing true will return 1 and false returns 0.

    Number('34.00') // 34
    Number('-34') // -34
    Number('123e5') // 12300000
    Number('123e-5') // 0.00123
    Number('999999999999') // 999999999999
    Number('9999999999999999') // 10000000000000000 (integer accuracy up to 15 digit)
    Number('0xFF') // 255
    Number('Infinity') // Infinity  

    Number('34px') // NaN
    Number('xyz') // NaN
    Number('true') // NaN
    Number('false') // NaN

    // cavets
    Number('    ') // 0
    Number('\t\t') // 0
    Number('\n\t') // 0

Answer 5

Why is jQuery's implementation not good enough?

function isNumeric(a) {
    var b = a && a.toString();
    return !$.isArray(a) && b - parseFloat(b) + 1 >= 0;
};

Michael suggested something like this (although I've stolen "user1691651 - John"'s altered version here):

function isNumeric(num){
    num = "" + num; //coerce num to be a string
    return !isNaN(num) && !isNaN(parseFloat(num));
}

The following is a solution with most likely bad performance, but solid results. It is a contraption made from the jQuery 1.12.4 implementation and Michael's answer, with an extra check for leading/trailing spaces (because Michael's version returns true for numerics with leading/trailing spaces):

function isNumeric(a) {
    var str = a + "";
    var b = a && a.toString();
    return !$.isArray(a) && b - parseFloat(b) + 1 >= 0 &&
           !/^\s+|\s+$/g.test(str) &&
           !isNaN(str) && !isNaN(parseFloat(str));
};

The latter version has two new variables, though. One could get around one of those, by doing:

function isNumeric(a) {
    if ($.isArray(a)) return false;
    var b = a && a.toString();
    a = a + "";
    return b - parseFloat(b) + 1 >= 0 &&
            !/^\s+|\s+$/g.test(a) &&
            !isNaN(a) && !isNaN(parseFloat(a));
};

I haven't tested any of these very much, by other means than manually testing the few use-cases I'll be hitting with my current predicament, which is all very standard stuff. This is a "standing-on-the-shoulders-of-giants" situation.

Answer 6

PFB the working solution:

 function(check){ 
    check = check + "";
    var isNumber =   check.trim().length>0? !isNaN(check):false;
    return isNumber;
    }