Random Number Generator for 3d6 or any number of dice for a given sided dice.

dice.js

// Random number from 1 to numberOfSides. 
function dx(numberOfSides) {
  return Math.floor(Math.random() * numberOfSides + 1);
}

let xDy = (n, f) => { let x = 0; while(n-- > 0) x += dx(f); return x};

// usage:   numDiceSideDrop(4,6,1)
// examples: 
//   numDiceSideDrop(4,6,1)    Roll four six sided dice and drop the lowest die: 
//   numDiceSideDrop(2,6,0,6)  Roll 2 six sided dice and add 6.  (2d6+6)
function numDiceSidesDrop(numberOfDice, sides, dropLowest = 0, flatAdd = 0) {
  const arr = new Array(numberOfDice).fill(numberOfDice).map(() => dx(sides));

  if (dropLowest > 0) {
    arr.sort((a,b) => (b - a)).length = (numberOfDice - dropLowest);
  }

  const tot = arr.reduce( (a, b) => (a + b) );
  return tot + flatAdd;
}