Roll D&D style dice.

common_golf.js

// hand golfed the most common dice rolls.  

function rnd(max, min) {
  return 0| (Math.random() * max) + 0|min;
}
var d20 = rnd.bind(0, 20, 1);
var d8 = rnd.bind(0, 8, 1)
var d6 = rnd.bind(0, 6, 1)
var d4 = rnd.bind(0, 4, 1);

dice.js

// Rolls a d20 dice string.
// Examples: 2d6, 1d20, 4d2+7, 1d10-2
function rollDice(dice) {
  const regexDice = /(\d*)d(\d+)([+-]?\d*)/;
  const match = regexDice.exec(dice);           // parse the dice string
  const max = parseInt(match[2], 10);           // number of sides on the dice.
  const modifier = parseInt(match[3], 10) || 0; // optional modifier post roll.
  let count = parseInt(match[1], 10) || 1;      // number of dice to roll.
  let result = 0;
  
  if (Number.isNaN(max)) {
    throw new Error('Number of sides on dice not specified. ' + dice);
  }
  
  // Roll each dice, adding the result.
  while (count--) {
    result += 0 | ((Math.random() * max) + 1)
  }
    
  // add the modifier
  result += modifier;
  return result;
}

dice_golf.js

// This is a hand golfed version of dice.js
// When compressed it is only 138 bytes.

// dice is a string in D&D style
// example: 2d6, 1d20, 4d2+7, 1d10-2
function rollDice(dice) {
  var match = /(\d*)d(\d+)([+-]?\d*)/.exec(dice);
  var numberOfDice = 0|match[1] || 1;
  var numberOfSides = 0|match[2];
  var modifier = 0|match[3];
  var result = 0;

  while (numberOfDice--) {
    result += 0| Math.random() * numberOfSides + 1;
  }

  result += modifier;
  return result;
}