Returns a item-to-subitem path strictly based on `index` access

IndexPath.jsx

// InDesign script (function) -- DRAFT [230706]
// Cf https://community.adobe.com/t5/indesign-discussions/indesign-script-keep-reference-of-nested-pageitem-in-group-while-duplicating-it/td-p/13917002

function indexPath(/*PageItem*/child,/*PageItem*/root,/*bool=0*/AS_SPEC,  a,t,p,rk,tk,pk,i,o,x,s)
//----------------------------------
// Assuming that `root` is a parent of `child` in the DOM hierarchy, returns
// a root-to-child path in the form of a `.<collection>[<index>]` sequence,
// each element being introduced by '.', e.g. `.textFrames[1].ovals[2].textFrames[0]`
// The result is then that `child === root.textFrames[1].ovals[2].textFrames[0]`
// ---
// Optionally, set `AS_SPEC` to true to get a "partial specifier" instead, that is,
// a string in the form `/text-frame[1]/oval[2]/text-frame[0]`. This option makes
// it easier to rebuild a clean DOM specifier relative to the `root` object, using
//    `mySpec = root.toSpecifier() + indexPath(child, root, true);`
// so that `resolve(mySpec)` will return the `child` object.
// ---
// => str [OK]  |  false|ERROR [KO]
{
	// const RE_DIGS = /\d+/g; // not used!

	if( !(child && child.isValid && 'Document'!=(tk=child.constructor.name)) ) return false;
	if( !(root && root.isValid && 'Document'!=(rk=root.constructor.name)) ) return false;
	
	for( a=[], t=child.getElements()[0] ; 'Document' != (pk=(p=t.parent).constructor.name) ; tk=pk, t=p )
	{
		// Translate singular class to collection property (roughly!)
		// E.g. TextFrame -> textFrames ; Rectangle -> rectangles ; MultiStateObject -> multiStateObjects ; etc
		// ---
		tk = tk.charAt(0).toLowerCase() + tk.slice(1) + 's';

		if( p.hasOwnProperty('parentTextFrames') )
		{
			// Current parent is a text object (Character) so `t` is anchored.
			// ---
			if( 1 === (o=p.parentTextFrames).length )
			{
				// Regular case -> Treat the parent frame as the actual 'parent'.
				p = o[0];
				pk = p.constructor.name; // Might be TextFrame or TextPath
				i = -1;                  // Flag -> compute the index
			}
			else
			{
				// 'Ghost' anchor -> May we use the parent story as a fallback?
				throw "Ghost anchor!";
			}
		}
		else
		{
			// Current parent is *probably* a page item of some kind.
			// ---
			i = t.hasOwnProperty('index') && t.index;
			if( false===i ) throw ("The .index property is unavailable in " + t);
		}

		if( !p.hasOwnProperty(tk) ) throw ("Unknown property: " + tk);
		
		if( 0 > i )
		{
			// The anchored object has no given index that we could trust,
			// let's try to find it 'manually'.
			// ---
			s = '|' + p[tk].everyItem().id.join('|') + '|';
			x = s.indexOf('|' + t.id + '|');
			i = 0 <= x && x && s.slice(1,x).split('|').length;
			( (o=false!==i&&p[tk][i]).isValid && t===o.getElements()[0] ) ||
			( o=false );
		}
		else
		{
			// Tests have shown that *in principle* t.index is relative
			// to `tk`, but it could be pageItems-relative in weird cases...
			// ---
			( (o=p[tk][i]).isValid && t===o.getElements()[0] ) ||
			( 'pageItems'!=tk && (o=p[tk='pageItems'][i]).isValid && t===o.getElements()[0] ) ||
			( o=false );
		}

		if( !o ) throw ("Cannot recover " + t + " from " + p);

		a.push( AS_SPEC ? p[tk][i].toSpecifier().split('/').pop() : (tk+'['+i+']') );

		// Note: pk==rk is a precondition, it should run a bit faster than p===root.
		// ---
		if( rk==pk && p===root )
		{
			// e.g `.textFrames[1].ovals[2].textFrames[0]` if !(AS_SPEC)
			//  or `/text-frame[1]/oval[2]/text-frame[0]`  if AS_SPEC
			s = AS_SPEC ? '/' : '.';
			return s + a.reverse().join(s);
		}
	}

	return false;
};


// Test me.
var root = app.activeDocument.groups[0];   // Take a group (other PageItem subclass would work as well)
var child = app.selection[0];              // The selection must be a PageItem, somewhere inside that group
var path = indexPath( child, root, true ); // The path looks like "/text-frame[1]/oval[0]/text-frame[0]"
var spec = root.toSpecifier() + path;      // Append the path to the root specifier.
var t = resolve(spec);
alert( t===child );                        // Should be true.