I know some mocha and I wrote a fun recursive change calculator

I know I’m “supposed to” use arrays and mutation but I went the
functional route.

cash.js

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+/* totalPayable = 210         // £2.10
+cashPaid     = 300         // £3.00
+dfference    =  90         // 90p
+change       = [50,20,20]  // 50p, 20p, 20p */ 
+
+var _ = require("underscore");
+var coins = [5000, 2000, 1000, 500, 200, 100, 50, 20, 10, 5, 2, 1];
+var C = {};
+
+C.getChange = function (totalPayable, cashPaid) {
+	'use strict';
+	// a function that consumes a value and an array of Coin sizes, in order from largest to smallest
+	// returns the minimal combination of coins for the given value
+	var getCoins = function (amount, coinsSoFar, coinsAvailable) {
+		var biggestCoin = coinsAvailable[0]; // the first Coin in the list will be the biggest.
+		if (amount == 0) { // we've already taken care of all the coins and they're in our array, just return it.
+			return coinsSoFar;
+		} 
+		else if (amount >= biggestCoin) { // we're definitely going to use the biggest coin, maybe bringing us to zero if it's equal to the amount.
+			return getCoins (amount - biggestCoin, coinsSoFar.concat([biggestCoin]), (coinsAvailable));
+		}
+				else { // the biggest coin is too big so throw it out and try again
+					return getCoins (amount, coinsSoFar, _.rest(coinsAvailable));
+				}
+		};
+		// now substract the amount, give getCoins an empty array and the constant "coins"
+	return getCoins (cashPaid - totalPayable, [], coins);
+};
+module.exports = C;

index.js

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+var C = require("./cash.js");
+
+console.log(C.getChange(121, 5000));