xfa.resolveNode("this.caption.value.#text").value - will give the caption of current field.
ur Code block will look like:
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xfa.host.messageBox(field.mandatoryMessage);
xfa.host.setFocus(field);
//I want to get the field object's caption text
var cval = xfa.resolveNode(field.name+".caption.value.#text").value
app.alert("CAPTION: "+cval);
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-Raghu.