Learn Practice Daily golf Reference
Vertical movement·Lesson 2 of 6·8 min·0/7 exercisesPro

Absolute jumps

Line numbers are page numbers: gg and G for the covers, 42G or :42↵ to turn to a line you were told. Then follow a stack trace and fix what it points at.

Scene

Thursday, 3:40 p.m. The stack trace in the chat: an order crashed while working out its delivery fee.

  • TypeError: Cannot read properties of undefined (reading 'lat')
  • at distanceKm (legacy.js:84)
  • at placeOrder (legacy.js:194)
  • at reorder (legacy.js:290)

Three line numbers, none of them near the screen, and the ruler only measures the page you're on. Books solved this centuries ago. They print the page number.

From memory

Warm-up from earlier chapters

Two from earlier chapters, from memory, before the new idea. Two wrong tries show the answer.

The gutter says 7 next to the line above you. Get there.
press the keys…2 keys
1 of 2
The gutter says 7 next to the line above you. Get there.

Covers

gg goes to the first line of the file, the front cover. G goes to the last line, the back cover. From anywhere, one or two keys, and they're the fastest way to get your bearings in a file you don't know.

gg
// legacy.js — Byte Burrito, 2019–present. Please don't add anything here.
// (Everyone has added something here.)
const TAX_RATE = 0.0875;
const MAX_DISTANCE_KM = 8;
const OPEN_HOUR = 11;
const CLOSE_HOUR = 23;
const RUSH_START = 12;
const RUSH_END = 14;
function isOpen(now) {
const hour = now.getHours();
G
// legacy.js — Byte Burrito, 2019–present. Please don't add anything here.
// (Everyone has added something here.)
const TAX_RATE = 0.0875;
const MAX_DISTANCE_KM = 8;
const OPEN_HOUR = 11;
const CLOSE_HOUR = 23;
const RUSH_START = 12;
const RUSH_END = 14;
function isOpen(now) {
const hour = now.getHours();
5G
// legacy.js — Byte Burrito, 2019–present. Please don't add anything here.
// (Everyone has added something here.)
const TAX_RATE = 0.0875;
const MAX_DISTANCE_KM = 8;
const OPEN_HOUR = 11;
const CLOSE_HOUR = 23;
const RUSH_START = 12;
const RUSH_END = 14;
function isOpen(now) {
const hour = now.getHours();
One press each from line 7: gg to the first line, G to the last, 5G to line 5.

Turn to a page

Put a line number in front of G and you turn straight to that page: 84G goes to line 84, wherever you are. Typing :84 and ↵ does exactly the same, as a command; use whichever your fingers prefer.

1// legacy.js — Byte Burrito, 2019–present. Please don't add anything here.
1// (Everyone has added something here.)
2
3const TAX_RATE = 0.0875;
4const MAX_DISTANCE_KM = 8;
5const OPEN_HOUR = 11;
6const CLOSE_HOUR = 23;
7const RUSH_START = 12;
8const RUSH_END = 14;
9
10function isOpen(now) {
11 const hour = now.getHours();
12 return hour >= OPEN_HOUR && hour < CLOSE_HOUR;
13}
14
15function isLunchRush(now) {
16 const hour = now.getHours();
17 return hour >= RUSH_START && hour < RUSH_END;
18}
19
20function formatPrice(cents) {
21 const dollars = Math.floor(cents / 100);
22 const rest = String(cents % 100).padStart(2, "0");
23 return "$" + dollars + "." + rest;
24}
25
26function subtotal(items) {
27 let sum = 0;
28 for (const item of items) {
29 sum += item.price * item.qty;
30 }
31 return sum;
32}
33
34function taxFor(amount) {
35 return Math.round(amount * TAX_RATE);
36}
37
38function totalFor(order) {
39 const base = subtotal(order.items);
40 const tax = taxFor(base);
41 const tip = order.tip || 0;
42 return base + tax + tip;
43}
44
45// DEAD CODE: the fax ordering line was switched off in 2020
46function faxOrder(order) {
47 const sheet = renderFaxSheet(order);
48 faxMachine.send(sheet, "+1-555-0100");
49 log("fax sent", order.id);
50 return true;
51}
52
53function validateOrder(order) {
54 if (!order.items || order.items.length === 0) return "empty";
55 if (order.items.some((i) => i.qty <= 0)) return "bad quantity";
56 if (!order.address) return "no address";
57 if (!isOpen(new Date())) return "closed";
58 return null;
59}
60
61function applyCoupon(order, code) {
62 const coupon = COUPONS[code];
63 if (!coupon) return order;
64 if (coupon.expires < Date.now()) return order;
65 const off = Math.round(subtotal(order.items) * coupon.percent);
66 return { ...order, discount: off };
67}
68
69function studentDiscount(order, user) {
70 if (!user.isStudent) return 0;
71 return Math.round(subtotal(order.items) * 0.1);
72}
73
74// DEAD CODE: loyalty points were replaced by the app in 2021
75function addLoyaltyPoints(user, order) {
76 const points = Math.floor(totalFor(order) / 100);
77 user.points = (user.points || 0) + points;
78 saveUser(user);
79 return points;
80}
81
82function distanceKm(a, b) {
83 const dLat = (b.lat - a.lat) * 111;
84 const dLng = (b.lng - a.lng) * 111 * Math.cos((a.lat * Math.PI) / 180);
85 return Math.sqrt(dLat * dLat + dLng * dLng);
86}
87
88function nearestRider(riders, address) {
89 let best = null;
90 for (const rider of riders) {
91 if (!rider.online) continue;
92 const d = distanceKm(rider.position, address.position);
93 if (d > MAX_DISTANCE_KM) continue;
94 if (!best || d < best.distance) best = { rider, distance: d };
95 }
96 return best;
97}
98
99function assignRider(order, riders) {
100 const match = nearestRider(riders, order.address);
101 if (!match) return refund(order, "No rider nearby, sorry!");
102 order.riderId = match.rider.id;
103 order.eta = Math.ceil(match.distance * 4) + 10;
104 notify(match.rider.phone, "New order: " + order.id);
105 return order;
106}
107
108function etaMinutes(order, now) {
109 const rush = isLunchRush(now) ? 10 : 0;
110 return order.eta + rush;
111}
112
113// DEAD CODE: the pager integration, before the riders had phones
114function pageRider(rider, order) {
115 const message = "ORDER " + order.id + " " + order.address.street;
116 pager.send(rider.pagerId, message.slice(0, 80));
117 return true;
118}
119
120function refund(order, reason) {
121 order.status = "refunded";
122 order.refundReason = reason;
123 payments.refund(order.paymentId, totalFor(order));
124 notify(order.customerPhone, "Refunded: " + reason);
125 return order;
126}
127
128function markDelivered(order, now) {
129 order.status = "delivered";
130 order.deliveredAt = now.toISOString();
131 const late = now - new Date(order.placedAt) > order.eta * 60000;
132 if (late) order.lateBy = Math.round((now - new Date(order.placedAt)) / 60000 - order.eta);
133 return order;
134}
135
136function kitchenTicket(order) {
137 const lines = order.items.map((i) => i.qty + " x " + i.name);
138 lines.unshift("ORDER " + order.id);
139 if (order.notes) lines.push("NOTE: " + order.notes);
140 return lines.join("\n");
141}
142
143function printTicket(order) {
144 const ticket = kitchenTicket(order);
145 printer.print(ticket);
146 log("printed", order.id);
147}
148
149// DEAD CODE: the old receipt printer, replaced by email receipts
150function printReceipt(order) {
151 const lines = order.items.map((i) => i.name + " " + formatPrice(i.price * i.qty));
152 lines.push("TOTAL " + formatPrice(totalFor(order)));
153 receiptPrinter.print(lines.join("\n"));
154}
155
156function emailReceipt(order, user) {
157 const body = renderReceipt(order);
158 mailer.send(user.email, "Your Byte Burrito receipt", body);
159}
160
161function menuItem(id) {
162 const item = MENU.find((m) => m.id === id);
163 if (!item) throw new Error("not on the menu: " + id);
164 return item;
165}
166
167function isSpicy(id) {
168 return menuItem(id).spicy === true;
169}
170
171function happyHourPrice(item, now) {
172 const hour = now.getHours();
173 if (hour >= 17 && hour < 19) return Math.round(item.price * 0.8);
174 return item.price;
175}
176
177function priceList(now) {
178 return MENU.map((item) => ({
179 id: item.id,
180 name: item.name,
181 price: formatPrice(happyHourPrice(item, now)),
182 }));
183}
184
185// DEAD CODE: the 2019 "burrito of the month", never shipped
186function burritoOfTheMonth(month) {
187 const picks = ["al-pastor", "veggie", "carnitas", "birria"];
188 return menuItem(picks[month % picks.length]);
189}
190
191function placeOrder(cart, user, now) {
192 const order = { id: newId(), items: cart.items, address: user.address, placedAt: now.toISOString() };
193 const fee = deliveryFee(order, distanceKm(KITCHEN, order.address));
194 const problem = validateOrder(order);
195 if (problem) throw new Error(problem);
196 order.discount = studentDiscount(order, user);
197 order.total = totalFor(order) - order.discount + fee;
198 return order;
199}
200
201function deliveryFee(order, km) {
202 const base = km <= 2 ? 0 : 1.5;
203 const perKm = km > 2 ? (km - 2) * 0.4 : 0;
204 const rush = isLunchRush(new Date()) ? 1 : 0;
205 return Math.round((base + perKm + rush) * 100) / 100;
206}
207
208function newId() {
209 return "BB-" + Date.now().toString(36) + "-" + Math.random().toString(36).slice(2, 6);
210}
211
212function log(event, ...details) {
213 console.log("[" + new Date().toISOString() + "] " + event, ...details);
214}
215
216function notify(phone, text) {
217 if (!phone) return;
218 sms.send(phone, text.slice(0, 160));
219}
220
221function saveUser(user) {
222 db.users.put(user.id, user);
223}
224
225function loadUser(id) {
226 return db.users.get(id);
227}
228
229function saveOrder(order) {
230 db.orders.put(order.id, order);
231 log("saved", order.id);
232}
233
234function loadOrder(id) {
235 return db.orders.get(id);
236}
237
238function ordersToday(now) {
239 const day = now.toISOString().slice(0, 10);
240 return db.orders.all().filter((o) => o.placedAt.startsWith(day));
241}
242
243function revenueToday(now) {
244 return ordersToday(now).reduce((sum, o) => sum + o.total, 0);
245}
246
247function busiestHour(now) {
248 const counts = new Array(24).fill(0);
249 for (const o of ordersToday(now)) counts[new Date(o.placedAt).getHours()]++;
250 return counts.indexOf(Math.max(...counts));
251}
252
253// DEAD CODE: the CSV export the accountant asked for, then never used
254function exportCsv(orders) {
255 const rows = orders.map((o) => [o.id, o.total, o.status].join(","));
256 rows.unshift("id,total,status");
257 return rows.join("\n");
258}
259
260function ridersOnline(riders) {
261 return riders.filter((r) => r.online).length;
262}
263
264function riderReport(riders, now) {
265 const online = ridersOnline(riders);
266 const rush = isLunchRush(now);
267 return { online, rush, short: rush && online < 6 };
268}
269
270function weatherDelay(weather) {
271 if (weather === "rain") return 8;
272 if (weather === "snow") return 20;
273 return 0;
274}
275
276function adjustEta(order, weather) {
277 order.eta = order.eta + weatherDelay(weather);
278 return order;
279}
280
281function cancelOrder(order, reason) {
282 if (order.status === "delivered") return order;
283 order.status = "cancelled";
284 return refund(order, reason);
285}
286
287function reorder(previous, user, now) {
288 const cart = { items: previous.items.map((i) => ({ ...i })) };
289 return placeOrder(cart, user, now);
290}
291
292function favourite(user) {
293 const counts = {};
294 for (const o of db.orders.byUser(user.id)) {
295 for (const i of o.items) counts[i.id] = (counts[i.id] || 0) + i.qty;
296 }
297 return Object.keys(counts).sort((a, b) => counts[b] - counts[a])[0];
298}
299
300module.exports = {
301 isOpen,
302 placeOrder,
303 assignRider,
304 markDelivered,
305 printTicket,
306 emailReceipt,
307 priceList,
308 revenueToday,
309 riderReport,
310 cancelOrder,
311 reorder,
312};
NORMALLoading the editor…1:1
Try it

Turn the pages

Targets anywhere in the file. When one is off the screen, the goal bar says its line number and the strip on the right marks it: G with the number in front.

1 of 6Move the cursor onto the on line 40, below the screen
1// legacy.js — Byte Burrito, 2019–present. Please don't add anything here.
1// (Everyone has added something here.)
2
3const TAX_RATE = 0.0875;
4const MAX_DISTANCE_KM = 8;
5const OPEN_HOUR = 11;
6const CLOSE_HOUR = 23;
7const RUSH_START = 12;
8const RUSH_END = 14;
9
10function isOpen(now) {
11 const hour = now.getHours();
12 return hour >= OPEN_HOUR && hour < CLOSE_HOUR;
13}
14
15function isLunchRush(now) {
16 const hour = now.getHours();
17 return hour >= RUSH_START && hour < RUSH_END;
18}
19
20function formatPrice(cents) {
21 const dollars = Math.floor(cents / 100);
22 const rest = String(cents % 100).padStart(2, "0");
23 return "$" + dollars + "." + rest;
24}
25
26function subtotal(items) {
27 let sum = 0;
28 for (const item of items) {
29 sum += item.price * item.qty;
30 }
31 return sum;
32}
33
34function taxFor(amount) {
35 return Math.round(amount * TAX_RATE);
36}
37
38function totalFor(order) {
39 const base = subtotal(order.items);
40 const tax = taxFor(base);
41 const tip = order.tip || 0;
42 return base + tax + tip;
43}
44
45// DEAD CODE: the fax ordering line was switched off in 2020
46function faxOrder(order) {
47 const sheet = renderFaxSheet(order);
48 faxMachine.send(sheet, "+1-555-0100");
49 log("fax sent", order.id);
50 return true;
51}
52
53function validateOrder(order) {
54 if (!order.items || order.items.length === 0) return "empty";
55 if (order.items.some((i) => i.qty <= 0)) return "bad quantity";
56 if (!order.address) return "no address";
57 if (!isOpen(new Date())) return "closed";
58 return null;
59}
60
61function applyCoupon(order, code) {
62 const coupon = COUPONS[code];
63 if (!coupon) return order;
64 if (coupon.expires < Date.now()) return order;
65 const off = Math.round(subtotal(order.items) * coupon.percent);
66 return { ...order, discount: off };
67}
68
69function studentDiscount(order, user) {
70 if (!user.isStudent) return 0;
71 return Math.round(subtotal(order.items) * 0.1);
72}
73
74// DEAD CODE: loyalty points were replaced by the app in 2021
75function addLoyaltyPoints(user, order) {
76 const points = Math.floor(totalFor(order) / 100);
77 user.points = (user.points || 0) + points;
78 saveUser(user);
79 return points;
80}
81
82function distanceKm(a, b) {
83 const dLat = (b.lat - a.lat) * 111;
84 const dLng = (b.lng - a.lng) * 111 * Math.cos((a.lat * Math.PI) / 180);
85 return Math.sqrt(dLat * dLat + dLng * dLng);
86}
87
88function nearestRider(riders, address) {
89 let best = null;
90 for (const rider of riders) {
91 if (!rider.online) continue;
92 const d = distanceKm(rider.position, address.position);
93 if (d > MAX_DISTANCE_KM) continue;
94 if (!best || d < best.distance) best = { rider, distance: d };
95 }
96 return best;
97}
98
99function assignRider(order, riders) {
100 const match = nearestRider(riders, order.address);
101 if (!match) return refund(order, "No rider nearby, sorry!");
102 order.riderId = match.rider.id;
103 order.eta = Math.ceil(match.distance * 4) + 10;
104 notify(match.rider.phone, "New order: " + order.id);
105 return order;
106}
107
108function etaMinutes(order, now) {
109 const rush = isLunchRush(now) ? 10 : 0;
110 return order.eta + rush;
111}
112
113// DEAD CODE: the pager integration, before the riders had phones
114function pageRider(rider, order) {
115 const message = "ORDER " + order.id + " " + order.address.street;
116 pager.send(rider.pagerId, message.slice(0, 80));
117 return true;
118}
119
120function refund(order, reason) {
121 order.status = "refunded";
122 order.refundReason = reason;
123 payments.refund(order.paymentId, totalFor(order));
124 notify(order.customerPhone, "Refunded: " + reason);
125 return order;
126}
127
128function markDelivered(order, now) {
129 order.status = "delivered";
130 order.deliveredAt = now.toISOString();
131 const late = now - new Date(order.placedAt) > order.eta * 60000;
132 if (late) order.lateBy = Math.round((now - new Date(order.placedAt)) / 60000 - order.eta);
133 return order;
134}
135
136function kitchenTicket(order) {
137 const lines = order.items.map((i) => i.qty + " x " + i.name);
138 lines.unshift("ORDER " + order.id);
139 if (order.notes) lines.push("NOTE: " + order.notes);
140 return lines.join("\n");
141}
142
143function printTicket(order) {
144 const ticket = kitchenTicket(order);
145 printer.print(ticket);
146 log("printed", order.id);
147}
148
149// DEAD CODE: the old receipt printer, replaced by email receipts
150function printReceipt(order) {
151 const lines = order.items.map((i) => i.name + " " + formatPrice(i.price * i.qty));
152 lines.push("TOTAL " + formatPrice(totalFor(order)));
153 receiptPrinter.print(lines.join("\n"));
154}
155
156function emailReceipt(order, user) {
157 const body = renderReceipt(order);
158 mailer.send(user.email, "Your Byte Burrito receipt", body);
159}
160
161function menuItem(id) {
162 const item = MENU.find((m) => m.id === id);
163 if (!item) throw new Error("not on the menu: " + id);
164 return item;
165}
166
167function isSpicy(id) {
168 return menuItem(id).spicy === true;
169}
170
171function happyHourPrice(item, now) {
172 const hour = now.getHours();
173 if (hour >= 17 && hour < 19) return Math.round(item.price * 0.8);
174 return item.price;
175}
176
177function priceList(now) {
178 return MENU.map((item) => ({
179 id: item.id,
180 name: item.name,
181 price: formatPrice(happyHourPrice(item, now)),
182 }));
183}
184
185// DEAD CODE: the 2019 "burrito of the month", never shipped
186function burritoOfTheMonth(month) {
187 const picks = ["al-pastor", "veggie", "carnitas", "birria"];
188 return menuItem(picks[month % picks.length]);
189}
190
191function placeOrder(cart, user, now) {
192 const order = { id: newId(), items: cart.items, address: user.address, placedAt: now.toISOString() };
193 const fee = deliveryFee(order, distanceKm(KITCHEN, order.address));
194 const problem = validateOrder(order);
195 if (problem) throw new Error(problem);
196 order.discount = studentDiscount(order, user);
197 order.total = totalFor(order) - order.discount + fee;
198 return order;
199}
200
201function deliveryFee(order, km) {
202 const base = km <= 2 ? 0 : 1.5;
203 const perKm = km > 2 ? (km - 2) * 0.4 : 0;
204 const rush = isLunchRush(new Date()) ? 1 : 0;
205 return Math.round((base + perKm + rush) * 100) / 100;
206}
207
208function newId() {
209 return "BB-" + Date.now().toString(36) + "-" + Math.random().toString(36).slice(2, 6);
210}
211
212function log(event, ...details) {
213 console.log("[" + new Date().toISOString() + "] " + event, ...details);
214}
215
216function notify(phone, text) {
217 if (!phone) return;
218 sms.send(phone, text.slice(0, 160));
219}
220
221function saveUser(user) {
222 db.users.put(user.id, user);
223}
224
225function loadUser(id) {
226 return db.users.get(id);
227}
228
229function saveOrder(order) {
230 db.orders.put(order.id, order);
231 log("saved", order.id);
232}
233
234function loadOrder(id) {
235 return db.orders.get(id);
236}
237
238function ordersToday(now) {
239 const day = now.toISOString().slice(0, 10);
240 return db.orders.all().filter((o) => o.placedAt.startsWith(day));
241}
242
243function revenueToday(now) {
244 return ordersToday(now).reduce((sum, o) => sum + o.total, 0);
245}
246
247function busiestHour(now) {
248 const counts = new Array(24).fill(0);
249 for (const o of ordersToday(now)) counts[new Date(o.placedAt).getHours()]++;
250 return counts.indexOf(Math.max(...counts));
251}
252
253// DEAD CODE: the CSV export the accountant asked for, then never used
254function exportCsv(orders) {
255 const rows = orders.map((o) => [o.id, o.total, o.status].join(","));
256 rows.unshift("id,total,status");
257 return rows.join("\n");
258}
259
260function ridersOnline(riders) {
261 return riders.filter((r) => r.online).length;
262}
263
264function riderReport(riders, now) {
265 const online = ridersOnline(riders);
266 const rush = isLunchRush(now);
267 return { online, rush, short: rush && online < 6 };
268}
269
270function weatherDelay(weather) {
271 if (weather === "rain") return 8;
272 if (weather === "snow") return 20;
273 return 0;
274}
275
276function adjustEta(order, weather) {
277 order.eta = order.eta + weatherDelay(weather);
278 return order;
279}
280
281function cancelOrder(order, reason) {
282 if (order.status === "delivered") return order;
283 order.status = "cancelled";
284 return refund(order, reason);
285}
286
287function reorder(previous, user, now) {
288 const cart = { items: previous.items.map((i) => ({ ...i })) };
289 return placeOrder(cart, user, now);
290}
291
292function favourite(user) {
293 const counts = {};
294 for (const o of db.orders.byUser(user.id)) {
295 for (const i of o.items) counts[i.id] = (counts[i.id] || 0) + i.qty;
296 }
297 return Object.keys(counts).sort((a, b) => counts[b] - counts[a])[0];
298}
299
300module.exports = {
301 isOpen,
302 placeOrder,
303 assignRider,
304 markDelivered,
305 printTicket,
306 emailReceipt,
307 priceList,
308 revenueToday,
309 riderReport,
310 cancelOrder,
311 reorder,
312};
NORMALLoading the editor…1/60.0s1:1

This exercise needs a keyboard. Read on, and play it at your computer.

Ruler or page number?

Both get you there; pick by what you know. Can you see the line? Read the ruler: 13j. Were you told the line, by a stack trace, an error or a teammate? Turn to the page: 214G. Your own line always shows its page number in the gutter, so you know where you are.

Check yourself

Page 12, or twelve on?

4// legacy.js — Byte Burrito, 2019–present. Please don't add anything here.
3// (Everyone has added something here.)
2
1const TAX_RATE = 0.0875;
5const MAX_DISTANCE_KM = 8;
1const OPEN_HOUR = 11;
2const CLOSE_HOUR = 23;
3const RUSH_START = 12;
4const RUSH_END = 14;
5
6function isOpen(now) {
7 const hour = now.getHours();
8 return hour >= OPEN_HOUR && hour < CLOSE_HOUR;
9}
10
11function isLunchRush(now) {
12 const hour = now.getHours();
13 return hour >= RUSH_START && hour < RUSH_END;
NORMAL
Keys pressed: 12G

The cursor is on line 5. You press 12G. Which line are you on?

Pick an answer (or press its number), then watch the engine prove it.
From memory

Page numbers, from memory

Press the keys.

To the first line of the file.
scroll here, then press the keys2 keys
1 of 4
To the first line of the file.
Challenge

Bug hunt

Twelve stops anywhere in legacy.js. Ruler for what you can see, page numbers for what you can't.

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This exercise needs a keyboard. Read on, and play it at your computer.

Boss round

The stack trace

Turn to each frame of the trace in turn; the goal bar gives the line number and what you find there. G with the number in front, except frame 3: type that one as a command, :290 and ↵. Then two more pages: the address check that should have caught this, and where it runs.

Pro
The rest of this lesson is in Pro

The first exercise of every lesson is free. Pro unlocks the rest, every track, for good.

See Pro

This exercise needs a keyboard. Read on, and play it at your computer.

Boss round

One line in the wrong place

The fix is a move, not a rewrite. On line 194, const fee = deliveryFee(…) runs before the address check. Move that line down so it sits under if (problem) throw …: cut the whole line, step down one, put it below.

Stuck? Show a hint
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The rest of this lesson is in Pro

The first exercise of every lesson is free. Pro unlocks the rest, every track, for good.

See Pro

This exercise needs a keyboard. Read on, and play it at your computer.

The reorder, againFinish the exercise above to see it.

Extra practice optional

The chapter is done. These rounds keep going as long as you like: stop whenever it stops being useful.

Endless

Page turner

Endless targets anywhere in the file. Ruler or page number: your call.

Pro
The rest of this lesson is in Pro

The first exercise of every lesson is free. Pro unlocks the rest, every track, for good.

See Pro

This exercise needs a keyboard. Read on, and play it at your computer.