Exact-change.js

// Create an object which hold the denominations and their values 建立一个json来将原来的英文表述转化为数字表述，方便以后的比较
var denom = [
  { name: 'ONE HUNDRED', val: 100.00},
  { name: 'TWENTY', val: 20.00},
  { name: 'TEN', val: 10.00},
  { name: 'FIVE', val: 5.00},
  { name: 'ONE', val: 1.00},
  { name: 'QUARTER', val: 0.25},
  { name: 'DIME', val: 0.10},
  { name: 'NICKEL', val: 0.05},
  { name: 'PENNY', val: 0.01}
];


function checkCashRegister(price, cash, cid) {
  var change = cash - price;//找零数额等于给的现金数目减去商品价格

  // Transform CID array into drawer object reduce方法是累加器，用于累加，这里就是计算抽屉里的总金额数
  //arr.reduce(callback,[initialValue]) 第二个参数是累加器的初始值，在这里初始值是total：0
  var register = cid.reduce(function(acc, curr) {
    acc.total += curr[1];
    acc[curr[0]] = curr[1];
    return acc;
  }, {total: 0});

  // Handle exact change 抽屉里的金额与需找零相等
  if (register.total === change) {
    return 'Closed';
  }

  // Handle obvious insufficent funds 抽屉里的金额不足
  if (register.total < change) {
    return 'Insufficient Funds';
  }

  // Loop through the denomination array 遍历开头创建的数组，
  var change_arr = denom.reduce(function(acc, curr) {
    var value = 0;
    // While there is still money of this type in the drawer
    // And while the denomination is larger than the change reminaing有这种类型的钱，并且这种类型的钱比需找要少。
    //比如需找30，20元面值的钱有1张，这就够了。
    //这里的 curr.name
    while (register[curr.name] > 0 && change >= curr.val) {  //这就是上面提到的30元比20元多的情况
      change -= curr.val;                                     //重新设定需找数额，这时候就是还差10元了，因为已经有1张20元
      register[curr.name] -= curr.val;                          //拿总金额数来减
      value += curr.val;

      // Round change to the nearest hundreth deals with precision errors
      change = Math.round(change * 100) / 100;
    }
    // Add this denomination to the output only if any was used.
    if (value > 0) {
        acc.push([ curr.name, value ]);
    }
    return acc; // Return the current Change Array
  }, []); // Initial value of empty array for reduce

  // If there are no elements in change_arr or we have leftover change, return
  // the string "Insufficient Funds"
  if (change_arr.length < 1 || change > 0) {
    return "Insufficient Funds";
  }

  // Here is your change, ma'am.
  return change_arr;
}

// test here
checkCashRegister(19.50, 20.00, [["PENNY", 1.01], ["NICKEL", 2.05], ["DIME", 3.10], ["QUARTER", 4.25], ["ONE", 90.00], ["FIVE", 55.00], ["TEN", 20.00], ["TWENTY", 60.00], ["ONE HUNDRED", 100.00]]);