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

How to find that a number is float or integer?

1.25 --> float  
1 --> integer  
0 --> integer  
0.25 --> float

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 2019
5 years after this answer was written, a solution was standardized in ECMA Script 2015. That solution is covered in this answer.

Stack Overflow Q#3885817, score: 1516