TypeScript is the way to go. It would be more sensible coming from the C# dev background to have the structured code even in JavaScript world which is highly loose scripting language but with huge power.

You need to clone the contents of the source array. [copy] method of NSMutableArray makes a reference copy with new array variable hence your content in backup array gets updated when you update source array. Use

NSMutableArray initWithArray

to create a clone of source array but with new reference pointer to it. Hope it helps.