How do I check that a number is float or integer?

check for a remainder when dividing by 1:

function isInt(n) {   return n % 1 === 0;}

If you don't know that the argument is a number you need two tests:

function isInt(n){    return Number(n) === n && n % 1 === 0;}function isFloat(n){    return Number(n) === n && n % 1 !== 0;}

Update 20195 years after this answer was written, a solution was standardized in ECMA Script 2015. That solution is covered in this answer.

Try these functions to test whether a value is a number primitive value that has no fractional part and is within the size limits of what can be represented as an exact integer.

function isFloat(n) {    return n === +n && n !== (n|0);}function isInteger(n) {    return n === +n && n === (n|0);}

There is a method called Number.isInteger() which is currently implemented in everything but IE. MDN also provides a polyfill for other browsers:

Number.isInteger = Number.isInteger || function(value) {  return typeof value === 'number' &&     isFinite(value) &&     Math.floor(value) === value;};

However, for most uses cases, you are better off using Number.isSafeInteger which also checks if the value is so high/low that any decimal places would have been lost anyway. MDN has a polyfil for this as well. (You also need the isInteger pollyfill above.)

if (!Number.MAX_SAFE_INTEGER) {    Number.MAX_SAFE_INTEGER = 9007199254740991; // Math.pow(2, 53) - 1;}Number.isSafeInteger = Number.isSafeInteger || function (value) {   return Number.isInteger(value) && Math.abs(value) <= Number.MAX_SAFE_INTEGER;};