Besides the fact you can normally buy replacement from ikea (or ebay), most of the stuff is fixable via epoxy and wood sawdust(or even cardboard pieces as filler), wood filler accomplishes the task as well. Brass inserts (again epoxied) + machine bolts can fix up any holes/stripped wood screws. As a fast fix insert tooth picks in the hole along with PVA.

Of course in many cases is easier (faster/cheaper) to replace the entire 'thing', altogether.