newbie needs help on javascript

[gencom97]

Hello. I hope I'm not asking a stupid question, but I'm new to javascript, so I would appreciate any advice. Here I'm writing a function that is called when a pull-down menu's option from an ElementById("kab") is changed (onChange). This function creates an ElementById ("kec") element at an ElementById ("kecamatan"). The problem is, when the user selects different options from the element ("kab"), it always create a new element ("kec") despite ("kec") already exists. How can I delete the previous element ("kec")? Can anyone help me? Thank you

 

function kabkec (form) 

{	

    	x = form.kab.selectedIndex;

y = form.kab.options[x].value;

a=document.getElementById("kecamatan");

b=document.createElement("select");

b.setAttribute("id","kec");

b.setAttribute("name","kec");

b.setAttribute("class","isi");

if (x!=0)

{

	c=document.createElement("option");

	d=document.createTextNode("Seluruh " + y);

	c.setAttribute("value","");

	c.appendChild(d);

	b.appendChild(c);

	a.appendChild(b);

}

switch (x)

{

	case 1:

	{

		Kodya = ["Danurejan","Gedongtengen","Gondokusuman","Gondomanan","Jetis","Kotagede","Kraton","Mantrijeron","Mergangsan","Ngampilan","Pakualaman","Tegalrejo","Umbulharjo","Wirobrajan"];

		for (i=0;i<14;i++)

		{

			c=document.createElement("option");

			d=document.createTextNode(Kodya[i]);

			c.setAttribute("value",Kodya[i]);

			c.appendChild(d);

			b.appendChild(c);

			a.appendChild(b);

		}

		break;

	}

	case 2:

	{

		Sleman = ["Berbah","Cangkringan","Depok","Gamping","Godean","Kalasan","Minggir","Mlati","Moyudan","Ngaglik","Ngemplak","Pakem","Prambanan","Seyegan","Sleman","Tempel","Turi"];

		for (i=0;i<17;i++)

		{

			c=document.createElement("option");

			d=document.createTextNode(Sleman[i]);

			c.setAttribute("value",Sleman[i]);

			c.appendChild(d);

			b.appendChild(c);

			a.appendChild(b);

		}

		break;

	}

	case 3:

	{

		Bantul = ["Bambanglipuro","Banguntapan","Bantul","Dlingo","Imogiri","Jetis","Kasihan","Kretek","Pajangan","Pandak","Piyungan","Pleret","Pundong","Sanden","Sedayu","Sewon","Srandakan"];

		for (i=0;i<17;i++)

		{

			c=document.createElement("option");

			d=document.createTextNode(Bantul[i]);

			c.setAttribute("value",Bantul[i]);

			c.appendChild(d);

			b.appendChild(c);

			a.appendChild(b);

		}

		break;

	}

	case 4:

	{

		Kulonprogo = ["Galur","Girimulyo","Kalibawang","Kokap","Lendah","Nanggulan","Panjatan","Pengasih","Samigaluh","Sentolo","Temon","Wates"];

		for (i=0;i<12;i++)

		{

			c=document.createElement("option");

			d=document.createTextNode(Kulonprogo[i]);

			c.setAttribute("value",Kulonprogo[i]);

			c.appendChild(d);

			b.appendChild(c);

			a.appendChild(b);

		}

		break;

	}

	case 5:

	{

		Gunungkidul = ["Gedangsari","Girisubo","Karangmojo","Ngawen","Nglipar","Paliyan","Panggang","Patuk","Playen","Ponjong","Purwosari","Rongkop","Saptosari","Semanu","Semin","Tanjungsari","Tepus","Wonosari"];

		for (i=0;i<18;i++)

		{

			c=document.createElement("option");

			d=document.createTextNode(Gunungkidul[i]);

			c.setAttribute("value",Gunungkidul[i]);

			c.appendChild(d);

			b.appendChild(c);

			a.appendChild(b);

		}

		break;

	}

}

return true;

}