JavaScript: is number

Deciding whether something is a number in JavaScript should be a straightforward type check (i.e., using the isNaN() function), but, as it turns out, it is a bit more tricky.

This is due to JavaScript’s forced conversion-to-number behavior. The often used methods, isNaN() and isFinite() can have confusing behavior and return incorrect answers.

Thankfully, ECMAScript 6 (JS language specification) was released with the intention of fixing these issues with Number.isNaN() and Number.isFinite(), along with making many other language clarifications and improvements.

Use the isNumeric() Function

Until that JS language version is the standard and used in browsers, we will need to be careful when checking if a value is a number.

The jQuery project has a tiny isNumeric() function implementation that fixes this odd JS behavior. There are some other implementations, but many of those fail in some of the JS type-casting-gotcha corner cases.

The isNumeric() function below is usable in your own code, even without jQuery.

var isNumeric = function(obj){
  return !Array.isArray( obj ) && (obj - parseFloat( obj ) + 1) >= 0;
}

isNumeric() Examples

Below are some examples of using isNumeric() with its boolean output:

isNumeric(10); // true
isNumeric(“0.75”); // true
isNumeric(“123e–2”); // true
isNumeric(–0xFFF); // true

isNumeric(Infinity); // false , use isFinite()
isNumeric(NaN); // false , use isNaN()
isNumeric([1]); // false
isNumeric(true); // false
isNumeric(“foo”); // false
isNumeric(null); // false
isNumeric(new Date()); // false