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

Because sometimes we want to convert from one type to another.

I don't understand - "(e.splice(), e)". How does the coma work here? This is the first time I see sth returned as (function, e). I'm confused.

Can someone explain to me how does "return" work in reduce()? I don't understand how it could loop like this. Isn't "return" like "break;" in loops? Shoudn't it "break;" after first iteration?

Yep, I meant in Chrome's console. I didn't try it in other browser. Do you have any idea why my & your previous code did fail in codewars? Not for the sake of completing a challenge, I'm just curious :)

Your code also fails. I tested my & your code in Chrome & it works 100% right. I think sth is wrong with this Kata, not our code.

I don't know if some is trolling me or I don't understand sth, but those tests seem to be bugged. There are tests, which make ZERO SENSE like - Tests 0,1,0,0 & it expects 2 instead 4. I passed all initial tests, but in the other phase, I failed massivly & all the tests I failed seem to be broken.