How to check whether a folder exists on JXA?

The last example was not exactly correct:

ObjC.import('Foundation')

var app = Application.currentApplication();
app.includeStandardAdditions = true;

var path = Path('/Users/yourname/Desktop/Test_Folder').toString();
app.displayAlert(path);

isDir=Ref();
$.NSFileManager.alloc.init.fileExistsAtPathIsDirectory(path, isDir);
app.displayAlert(isDir[0]);

var app = Application.currentApplication()
app.includeStandardAdditions = true


var path = Path('/Users/yourname/Desktop/Test_Folder');
var finderApp = Application("Finder");
var status = finderApp.exists(path);
app.displayAlert(status);

This is the simplest approach. It can also be done as:

ObjC.import('Foundation')

var app = Application.currentApplication()
app.includeStandardAdditions = true

var path = Path('/Users/yourname/Desktop/Test_Folder');

isDir=Ref()
$.NSFileManager.alloc.init.fileExistsAtPathIsDirectory(path, isDir);
app.displayAlert(isDir[0]);

In the latter case, it will be true if the object at the end of the path is a directory, and it exists, otherwise false is returned.