javascrip to count length of array on xhtml

javascrip to count length of array on xhtml

i am looking for how can i code javascrip to count one array direct on xhml.
Example, I have a piece of code xhtml
***<p:selectManyCheckbox id="select4" value="#{bean.arrayString}">
<f:selectItems value="#{func_InvPlanNewFixedAssets.listProgram}"
var="prog" itemLabel="#{prog.progName}" itemValue="#{prog.progId}"
/>
</p:selectManyCheckbox>***
So from code above how i can count length of value = "#bean.arrayString}"
using javascrip?
Example when firt load value of bean.arrayString have length is 0 and when
i checked a box it increase length = 1, so if i am using javascript, how ?
PLEASE HELP ME!
THANK ALL FOR HELP!