Check whether variable is number or string in Java

Question

Does anyone know how can I check whether a variable is a number or a string in JavaScript?

Answer 1

the best way i found which also thinks of positive and negative numbers is from : O'Reilly Javascript and DHTML Cookbook :

function isNumber(elem) {
var str = elem.value;
var oneDecimal = false;
var oneChar = 0;
// make sure value hasn't cast to a number data type
str = str.toString( );
for (var i = 0; i < str.length; i++) {
    oneChar = str.charAt(i).charCodeAt(0);
    // OK for minus sign as first character
    if (oneChar =  = 45) {
        if (i =  = 0) {
            continue;
        } else {
            alert("Only the first character may be a minus sign.");
            return false;
        }
    }
    // OK for one decimal point
    if (oneChar =  = 46) {
        if (!oneDecimal) {
            oneDecimal = true;
            continue;
        } else {
            alert("Only one decimal is allowed in a number.");
            return false;
        }
    }
    // characters outside of 0 through 9 not OK
    if (oneChar < 48 || oneChar > 57) {
        alert("Enter only numbers into the field.");
        return false;
    }
}
return true;

}

Answer 2

Since ES2015 the correct way to check if a variable holds a valid number is:

Number.isFinite(value)

Examples:

Number.isFinite(Infinity)   // false
Number.isFinite(NaN)        // false
Number.isFinite(-Infinity)  // false

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

Number.isFinite('0')        // false
Number.isFinite(null)       // false

Answer 3

Here's an approach based on the idea of coercing the input to a number or string by adding zero or the null string, and then do a typed equality comparison.

function is_number(x) { return x === x+0;  }
function is_string(x) { return x === x+""; }

For some unfathomable reason, x===x+0 seems to perform better than x===+x.

Are there any cases where this fails?

In the same vein:

function is_boolean(x) { return x === !!x; }

This appears to be mildly faster than either x===true || x===false or typeof x==="boolean" (and much faster than x===Boolean(x)).

Then there's also

function is_regexp(x)  { return x === RegExp(x); }

All these depend on the existence of an "identity" operation particular to each type which can be applied to any value and reliably produce a value of the type in question. I cannot think of such an operation for dates.

For NaN, there is

function is_nan(x) { return x !== x;}

This is basically underscore's version, and as it stands is about four times faster than isNaN(), but the comments in the underscore source mention that "NaN is the only number that does not equal itself" and adds a check for _.isNumber. Why? What other objects would not equal themselves? Also, underscore uses x !== +x--but what difference could the + here make?

Then for the paranoid:

function is_undefined(x) { return x===[][0]; }

Answer 4

Simply use

myVar.constructor == String

or

myVar.constructor == Number

if you want to handle strings defined as objects or literals and saves you don't want to use a helper function.