Hi Ufuk,
startsWith was initially defined in the ECMAScript 6 specification, and our Cloud Code engine, RhinoJS has not upgraded to ECMA6 as of yet.
Alternatives to startsWith include using the substring or lastIndexOf functions, as well as defining a regex for specific string patterns.
For example:
if (myString.substring(0, input.length) === input)
or
if (myString.lastIndexOf(input, 0) === 0)
-Pádraig
Ufuk Coban