Iterating through measure by measure?

Element.MEASURE will always return the same value. It is the numerical representation of the Type of Element. (so all measures are of the type Element.MEASURE)

To find out what you can call and or which properties exist: Open up the Plugin Creator and in there open up the Manual under its Help menu. You'll find that the no() function doesn't exist there, meaning it is not available to a plugin.

For your direct question, I've created this small sample plugin that counts measures whilst walking over the elements in voice 1 of the top staff of a score.

import QtQuick 2.0
import MuseScore 1.0

MuseScore {
      menuPath: "Plugins.pluginName"
      onRun: {
            if (!curScore) {
                  console.log(qsTranslate("QMessageBox", "No score open.\nThis plugin requires an open score to run.\n"));
                  Qt.quit();
            }
            var internalMeasureNumber = -1; //we will use this to track the current measure number
            var currentMeasure = null; //we use this to keep a reference to the actual measure, so we can know when it changes

            //get a cursor to be able to run over the score
            var cursor = curScore.newCursor();
            cursor.rewind(0);
            cursor.voice = 0;
            cursor.staffIdx = 0;

            while (cursor.segment) {
                  if (cursor.measure != currentMeasure) {
                        //we moved into a new measure
                        internalMeasureNumber++;
                        currentMeasure = cursor.measure;
                  }
                  cursor.next();
            }
            console.log('Found ' + (internalMeasureNumber + 1) + ' measures.');
            Qt.quit();
      }
}