• declare your variables
• do not modify your inputs
• the IIFE would be better expressed as a recursive function returning a ( the ) result
• factor out things like sum
• express your intent more clearly in your code
• using splice may seem attractive, but it's generally slow; try avoiding it

It's not as performant as it could be, but it has more or less the right asymptotic performance. It's not as maintainable as it could be ( and this is bigger than you possibly think ), but .. it works.