Select Numbers From List And Add To Text Box

I want to select numbers from a list box and add it to a text box with comma separated. my java script code does not work into browser but work fine into eclipse jsp view.

function passingNumbersToTextArea(numToCall,allNum)
{
var numFromList=document.getElementById(allNum);
var numToTextArea=document.getElementById(numToCall);
if(numToTextArea.value.search(numFromList.value)!=-1)//if duplicate Number is exist
{
alert("Duplicate Number Can not be added :");
return false;
}
else
{
if(numToTextArea.value.length!=0)
{
// If textarea has value than it added another value with comma seperated
numToTextArea.value=numToTextArea.value+','+numFromList.value;
return false;
}
else
{
numToTextArea.value=numFromList.value;
return false;
}
}
}

jsp code
<h:inputTextarea cols="10" rows="4" id="numberToCall" required="true"
value="#{conferenceCall.numberToCall}" styleClass="form_input_box" />
<h:selectOneListbox styleClass="form_selectmenu" id="allNumbers" size="5" onclick="passingNumbersToTextArea('numberToCall','allNumbers');">
<f:selectItem itemValue="9971701077" itemLabel="Sharad : 9971701077"/>
<f:selectItem itemValue="9990102381" itemLabel="Saurabh : 9990102381"/>
</h:selectOneListbox>

javascript debugger shows error
Error ``numToTextArea is null'' [x-] in file ``http://localhost:8888/SparkServiceProvisioningSystem/home.jsf'', line 21, character 0.
Exception ``TypeError: numToTextArea is null'' thrown from function passingNumbersToTextArea(allNum=string:"allNumbers", numToCall=string:"numberToCall") in <http://localhost:8888/SparkServiceProvisioningSystem/home.jsf> line 21.
[e] message = [string] "numToTextArea is null"
Exception ``TypeError: numToTextArea is null'' thrown from function onclick(event=MouseEvent:{0}) in <http://localhost:8888/SparkServiceProvisioningSystem/home.jsf> line 1.
[e] message = [string] "numToTextArea is null"
Error ``TypeError: numToTextArea is null'' [x-] in file ``http://localhost:8888/SparkServiceProvisioningSystem/home.jsf'', line 21, character 0.