(source: https://jsfiddle.net/jyy44uh9/20/)

fiddle.html

  <div id="qunit"></div>
  <div id="qunit-fixture"></div>
 

fiddle.js

function splitHitDice(input) {
	let dices = input.toLowerCase().split('d');
  return [...Array(parseInt(dices[0]))].map(_ => parseInt(dices[1]));
}

function calcHitDice(input) {
 return [].concat.apply([], input.replace(' ','').split('+').map(splitHitDice));
}

QUnit.test( "1d6", function( assert ) {
  assert.deepEqual(calcHitDice("1d6"), [6], "Passed!" );
});

QUnit.test( "1d8", function( assert ) {
  assert.deepEqual(calcHitDice("1d8"), [8], "Passed!" );
});

QUnit.test( "1D8", function( assert ) {
  assert.deepEqual(calcHitDice("1D8"), [8], "Passed!" );
});

QUnit.test( "2d8", function( assert ) {
  assert.deepEqual(calcHitDice("2d8"), [8,8], "Passed!" );
});

QUnit.test( "3d8", function( assert ) {
  assert.deepEqual(calcHitDice("3d8"), [8,8,8], "Passed!" );
});

QUnit.test( "1d6+3d8", function( assert ) {
  assert.deepEqual(calcHitDice("1d6+3d8"), [6,8,8,8], "Passed!" );
});

QUnit.test( "2d6+3d8", function( assert ) {
  assert.deepEqual(calcHitDice("2d6+3d8"), [6,6,8,8,8], "Passed!" );
});

QUnit.test( "1d4+2d6+3d8", function( assert ) {
  assert.deepEqual(calcHitDice("1d4+2d6+3d8"), [4,6,6,8,8,8], "Passed!" );
});

QUnit.test( "1d6 + 3d8", function( assert ) {
  assert.deepEqual(calcHitDice("1d6 + 3d8"), [6,8,8,8], "Passed!" );
});