So the line in your code:
…sets the value of a to 1, the value of b to 2, and the value of arr to… nothing at all. I honestly don’t see HOW this may have passed. running it in the console directly, or on jsfiddle, it fails. arr is returning as  rather than a populated array.
The issue is, by hardcoding in [1,2], you have successfully disallowed any other values to be passed in. Your function is being given an array, passed as a param
list – yet you never actually use that. You need to use the rest operator rather than using shift() in this challenge. Have you used it in the right place?