The Josephus problem in JavaScript
You can easily find the next death-index with the modulus operation: next = (current + space) % totalStillAlive . No need for fancy loops (see end of answer).
Instead of making an array of arrays (btw, don't create js arrays with new Array, use the array literal []), simply store an array of original indexes, and splice from that on each around until there are no more people left.
The way you're extracting the data is awkward, specifically the spacer definition - why not define it right off the bat as parseInt(...) - 1 ?
It's probably not very important with this input size, but the synchronous line reading is frowned upon; you should use the asynchronous versions unless there's a reason not to.
if (line != "") can just be if (line)
With #1 you don't need regroup, but your forEach loop is an implementation filter (docs)


(as a continuation of #2:

var person = [];
person = new Array(i.toString(), 1);


I don't get what went through your mind at the time, but why not just people[i] = [i, 1] ?)

So, your main algorithm can be simplified to 1 nice loop instead of 3 (discounting the instantiation loop):

function josephus (n, interval) {
    var people = [],
        deaths = [];
    for (var i = 0; i &lt; n; i += 1) {
        people[i] = i;
    }

    var idx = 0,
        len = people.length;
    while (len = people.length) {
        idx = (idx + interval) % len;
        deaths.push(people[idx]);
        people.splice(idx, 1);
    }

    return deaths;
}