There is no way to force garbage collection at a specific time in JavaScript. Each JavaScript engine has its own way of handling that.
What you can do, though, is make sure your objects aren't being referenced anywhere. Setting them to null or deleting them are actually doing the same thing (delete never deletes the object - it only removes a reference to it). When garbage collection does run, it checks to see if there are any references to a given object - then, if not, it removes it from memory. This is where memory leaks occur (a JavaScript object is referencing a DOM object, and vice-versa).
Make sure you remove all references and you'll be fine.
