We want for..in and for..each..in constructs to work for Java arrays, lists and maps (see JDK-8022903). It'd be nice if Array.prototype's iterator functions like forEach, map, filter etc. work on Java arrays and lists. Per ECMAScript spec. these are 'generic' functions and can work for host objects other than script arrays, script array-like objects. Already we support cross-global objects (script object mirrors) already.