Build a Xbonacci function that takes a signature of X elements - and remember each next element is the sum of the last X elements - and returns the first n elements of the so seeded sequence.

xbonacci.js

// This solves the problem with last and previous digit addition.
function Xbonacci(signature, n){
  var totalCallsToMake = n - signature.length;
  var tempArr = signature.slice(0);
  const reducer = (accumulator, currentValue) => accumulator + currentValue;
    // Looping through.
  for (var i=1; i<=totalCallsToMake; i++) {
    var lastTwoEl = tempArr.slice(-2);
    tempArr.push(lastTwoEl.reduce(reducer));
  }
  return tempArr;
}
Xbonacci([0,0,0,0,1],10);

// This solves the problem with last n digits.
function Xbonacci(signature, n){
  var totalCallsToMake = n - signature.length;
  var tempArr = signature.slice(0);
  const reducer = (accumulator, currentValue) => accumulator + currentValue;
    // Looping through.
  for (var i=1; i<=totalCallsToMake; i++) {
    tempArr.push(tempArr.reduce(reducer));
  }
  return tempArr;
}
Xbonacci([0,1],10);