You are using javascript. you can split it on "." then count the length of the array which should be 2 or less.

var x = '12.34.56';
var arr = x.split(".")
var number_of_dots = arr.length - 1
console.log(number_of_dots)

I think you can check if the number is Integer or float with this sample function

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

and write catch, so if had error the entered number is not valid for you calculator