Relative jumps
The gutter is a ruler measuring from your line: read the distance and jump, 13j, 7k, even on a long file.
Scene
Thursday, 3:05 p.m. Somebody asks where the delivery fee is calculated. Nobody knows, so it's in legacy.js. You open it, and it scrolls, and scrolls: 313 lines, a comment at the top that says Please don't add anything here, and seven years of people adding things there.
Reading it is like reading a long book nobody wanted to write. The mouse is still in the drawer.
From memory
Warm-up from earlier chapters
Two from earlier chapters, from memory, before the new idea. Two wrong tries show the answer.
Delete this line and the next two.
press the keys…3 keys
1 of 2
Delete this line and the next two.
The ruler reaches further
The gutter is still the ruler from day one, laid down the margin and measuring from your line. On a long file its numbers get bigger, and that's fine: a function 13 lines down says 13, so 13j lands on it. Three keys, however far the ruler reaches.
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 "$" + dollars + "." + rest;
24}
25
26function subtotal(items) {
27 let sum = 0;
28 for (const item of items) {
29 sum += item.price * item.qty;
30 }
31 return sum;
32}
33
34function taxFor(amount) {
35 return Math.round(amount * TAX_RATE);
36}
37
38function totalFor(order) {
39 const base = subtotal(order.items);
NORMALLoading the editor…1:1
Try it
Read the ruler
Every target is on screen. Read its distance off the gutter and jump in one go: 13j, 7k.
1// legacy.js — Byte Burrito, 2019–present. Please don't add anything here.
1// (Everyone has added something here.)
2
3const TAX_RATE = 0.0875;
4const MAX_DISTANCE_KM = 8;
5const OPEN_HOUR = 11;
6const CLOSE_HOUR = 23;
7const RUSH_START = 12;
8const RUSH_END = 14;
9
10function isOpen(now) {
11 const hour = now.getHours();
12 return hour >= OPEN_HOUR && hour < CLOSE_HOUR;
13}
14
15function isLunchRush(now) {
16 const hour = now.getHours();
17 return hour >= RUSH_START && hour < RUSH_END;
18}
19
20function formatPrice(cents) {
21 const dollars = Math.floor(cents / 100);
22 const rest = String(cents % 100).padStart(2, "0");
23 return "$" + 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.
Only what's on the page
A ruler only covers the page that's open: the gutter shows distances for the lines on screen, nothing further. For a line beyond the bottom you'd be guessing. The next lesson has the page numbers for that.
Check yourself
Past the edge
4const TAX_RATE = 0.0875;
3const MAX_DISTANCE_KM = 8;
2const OPEN_HOUR = 11;
1const CLOSE_HOUR = 23;
5const RUSH_START = 12;
1const RUSH_END = 14;
NORMAL
Keys pressed: 13k
The cursor is on line 5. You press 13k. Where does it end up?
Pick an answer (or press its number), then watch the engine prove it.
From memory
Read the ruler, from memory
Press the keys.
The gutter says 12 next to the line below you. Get there.
scroll here, then press the keys3 keys
1 of 3
The gutter says 12 next to the line below you. Get there.
Challenge
Section tour
Twelve stops on screen in legacy.js, further apart as you go, above you or below. Nobody tells you which way now: read the ruler, pick j or k, jump.
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
The wiki tour
The team wiki has a "tour of legacy.js" for new developers: six functions near the top, in order, with a line on what each one does. You're reading it for one thing: which of them works out the delivery fee? Visit each with one counted j or k, reading the ruler as you go.
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.
Extra practice optional
The chapter is done. These rounds keep going as long as you like: stop whenever it stops being useful.
Endless
Ruler practice
Endless targets on screen, further away every level.
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.