I guess I'm far too late but still will reply for future reader.
The first argument to the reduce callback is "accumulator" which is a variable that is what was returned from the function on the previous run.
If it's the first run of the function then accumulator will contain a first element of the array or the user specified value.
For example we can sum everything in array via [1,2,3].reduce((a,x) => a+x) //this uses a fat arrow syntax, that is left as an excercise to the reader

https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array/reduce

We don't want to return the result of the splice but the value of e after the splice.
See https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/Comma_Operator