in Javascript, exist a function that return only the fractional part of a number, like Math dot something…. ? How can I get it? [duplicate]

function(number, divisor){  

   //and return to me only the fractional part.

}

example : 45 , 20 = 2.25, I need return 0.25 to up, 0.3.

Sure, I can do many calculation inside and return what I need, I just want to know if have some way efficent, like Math.ceils and others functions…

thanks