xml javascript

i have an xml schema i am parsing javascript following make-up an vigilant pass backend servers.

the schema like

<complextype name='test'>
    <sequence>  
        <element name='testfield' type='string'/>
        <element name='typespecificsetting' type='tns:typespecific'/>
    </sequence> 
</complextype>
<complextype name="typespecific">
    <choice>   
         <element name='a' type='tns:atype'/>
         <element name='b' type='tns:btype'/>
         <element name='c' type='tns:ctype'/>
         <element name='d' type='tns:dtype'/>
    </choice>
</complextype>

<complextype name="atype">
    <element name='testatypefield' type='string'/>
</complextype>

<complextype name="btype">
     <element name='testbtypefield' type='string'/>
</complextype>

i am reading xml schema following perplexing make ask object.

request = { 
    testfield:  t1,
    typespecificsetting: t2
}

how i make ask vigilant choice? depending on type, i have container presumably atype btype ctype dtype objects? i grasp this?