Answer

orderBook.js

@@ -0,0 +1,48 @@
+
+function reconcileOrder(existingBook, incomingOrder) {
+  // default switch case key
+  let key = 'a'
+  // matching order index variable
+  let i = -1
+  // callback function for findIndex method which sets conditions for matching orders in existing book
+
+  const buyEqualOrder = order => order.type === 'buy' && order.price >= incomingOrder.price
+
+  // Finds matching order index in existing book
+
+  if (existingBook.length) { i = existingBook.findIndex(buyEqualOrder) }
+
+  // switches handling of orders in special scenarios
+
+  if (i >= 0 && incomingOrder.quantity <= existingBook[i].quantity) { key = 'b' }
+  if (i >= 0 && incomingOrder.quantity > existingBook[i].quantity) { key = 'c' }
+
+  switch (key) {
+    // adds an order to the book when the book is empty, or no matching order
+    case 'a':
+      existingBook.push(incomingOrder)
+
+      return existingBook
+    // fulfills an order and removes matching order with same quantity, or reduces when larger quantity
+    case 'b':
+      existingBook[i].quantity -= incomingOrder.quantity
+      if (existingBook[i].quantity > 0) { existingBook.push(existingBook.splice((i), 1)[0]) }
+      existingBook = existingBook.filter(orders => orders.quantity > 0)
+
+      return existingBook
+    // all the rest of the special cases (uses recursion to fullfill order with to existing orders)
+    case 'c':
+      incomingOrder.quantity -= existingBook[i].quantity
+      existingBook[i].quantity = 0
+      existingBook = existingBook.filter(orders => orders.quantity > 0)
+      if (existingBook.findIndex(buyEqualOrder) >= 0) {
+        existingBook = reconcileOrder(existingBook, incomingOrder)
+      }
+      else {
+        existingBook.push(incomingOrder)
+      }
+
+      return existingBook
+  }
+}
+module.exports = reconcileOrder

tests.js

@@ -91,7 +91,7 @@ describe('Order Book', () => {
       expect(updatedBook).to.deep.equal([{ type: 'sell', quantity: 12, price: 6950 }, { type: 'sell', quantity: 5, price: 6150 }])
     })
 
-    it.skip('Extra Credit: it fulfills a mismatched order when both parties benefit', () => {
+    it('Extra Credit: it fulfills a mismatched order when both parties benefit', () => {
       const existingBook = [{ type: 'buy', quantity: 15, price: 6000 }, { type: 'sell', quantity: 12, price: 6950 }]
       const incomingOrder = { type: 'sell', quantity: 15, price: 5900 }
 
@@ -100,7 +100,7 @@ describe('Order Book', () => {
       expect(updatedBook).to.deep.equal([{ type: 'sell', quantity: 12, price: 6950 }])
     })
 
-    it.skip('Extra Credit: it does not fulfill a mismatched order when it does not benefit both parties', () => {
+    it('Extra Credit: it does not fulfill a mismatched order when it does not benefit both parties', () => {
       const existingBook = [{ type: 'buy', quantity: 15, price: 5900 }, { type: 'sell', quantity: 12, price: 6950 }]
       const incomingOrder = { type: 'sell', quantity: 15, price: 6000 }