How can I do something similar to slideToggle() without JQuery?

[narutodude000]

I want to use something like slideToggle() in JQuery for my dropdown menu, but I do not want to load JQuery just for the one menu. How can I remake the function with just Javascript? Does anyone know of such a function?

[narutodude000]

I wrote this function, it's glitchy at times. How can I improve it?
"f" is the menu. "f+'content'" is the dropdown part of the menu.

function slideToggle(f){
if(typeof f=='undefined')
return;

fo=document.getElementById(f);
o=document.getElementById(f+'content')
 if(o.style.display!='block'){
    fo.setAttribute('class','open');
    o.style.opacity=0;
    o.style.display='block';
    o.style.height='auto';
    if(typeof h!='undefined')
      o.style.height=h;
    else
      h=o.offsetHeight;
    o.style.height=0;
    r=0;i=0;t=0;
    var int=setInterval(function(){
      o.style.height=i+'px';
      if(t<100){
        o.style.opacity=t;
        t=t+5;
      }
      i=i+r;r++;
      if(i>h){
        o.style.height=h;
        clearInterval(int);
      }
    },20);
 } else {
    r=0;i=o.offsetHeight;
    var int=setInterval(function(){
      o.style.height=i+'px';
      o.style.opacity=i/100;
      i=i-r;r++;
      if(i<1){
        clearInterval(int)
        o.style.display='none';
        fo.removeAttribute('class');
      }
    },20);
 }
}