You could also make several calls in one go:
window.AppInventor.setWebViewString('[{"current": "' + document.querySelectorAll('span.current')[0].innerHTML + '"}'+ ',' + '{"compare": "' + document.querySelectorAll('span.compare')[0].innerHTML + '"}]')
[{"current": "Rs. 699.00"},{"compare": "Rs. 1,299.00"}]