neat challenge I just solved, and was amazed at the simplicity of it due to the quirkiness of js, pic related
// mini-test track console log'd for your convenience
console.log(compress([1,1,2,2,2,3,1,1]))
// [1,2,3,1]
console.log(compress(["a","a","b","a","a","a"]))
// ["a","b","a"]
console.log(compress([]))
// []