Working with legacy code

A payments ledger splits a bill between payers. Today's code, which no test covers:

In the REPL:

> splitPence(900, 3)
[ 300, 300, 300 ]
> splitPence(1002, 3)
[ 334, 334, 334 ]
> splitPence(1001, 3)
[ 333, 333, 333 ]

Finance's rule is that the shares add up to the bill, any spare pennies going to the first payers, and finance has its own ticket open to make the code follow it. Kofi is about to refactor splitPence in small steps. Before step 1 he writes one test of splitPence(1001, 3).

A streaming service's nine-year-old royaltyFor(play) is called from 31 places in its code. A new royalty engine, built and tested on its own, handles podcast plays. Music plays must stay on the old code for now, and other kinds of play will move later.

A chat app's renderMessage(text) is 2,000 lines long, has no tests and no spec, and its author has left. Lena must change how it shows links, and before she touches it she will write characterisation tests.

A box office's booking fee lives in one module, and two modules import it:

// checkout/total.mjs
import { bookingFee } from '../fees/booking-fee.mjs';
export function totalPence(order) {
  return order.seats * order.seatPence + bookingFee(order);
}

// receipts/receipt.mjs
import { bookingFee } from '../fees/booking-fee.mjs';
export function feeLine(order) {
  return `Booking fee: ${bookingFee(order)}p`;
}

A new fee engine is ready for group orders, of eight seats or more. routed sends a group order to it and any other order to today's sum:

// fees/group-fee.mjs
export const groupFee = (order) => 300 + 20 * order.seats;

// fees/routed.mjs
import { groupFee } from './group-fee.mjs';
export const routed = (order) => (order.seats >= 8 ? groupFee(order) : 150 + 40 * order.seats);

Ravi must send group orders to it wherever the fee is charged or shown, while every other order keeps the fee it pays today. At start-up, app.mjs runs once, before any order is taken.

A ledger's monthly account fee, in code nobody on the team wrote, and its tests:

// ledger/monthly-fee.test.mjs
import { test } from 'node:test';
import assert from 'node:assert/strict';
import { monthlyFeePence } from './monthly-fee.mjs';

test('a low balance pays 400p', () => assert.equal(monthlyFeePence({ type: 'current', balancePence: 5000, openedYear: 2019 }), 400));
test('100,000p or more pays nothing', () => assert.equal(monthlyFeePence({ type: 'current', balancePence: 100000, openedYear: 2019 }), 0));
test('just under the threshold pays 400p', () => assert.equal(monthlyFeePence({ type: 'savings', balancePence: 99999, openedYear: 2021 }), 400));
test('a large old balance pays nothing', () => assert.equal(monthlyFeePence({ type: 'savings', balancePence: 250000, openedYear: 2008 }), 0));
test('an empty account pays 400p', () => assert.equal(monthlyFeePence({ type: 'current', balancePence: 0, openedYear: 2024 }), 400));
test('an old current account pays 400p', () => assert.equal(monthlyFeePence({ type: 'current', balancePence: 20000, openedYear: 2005 }), 400));

The bank has opened no student account since 2016, so Dee wants to delete the if on lines 3–5 as dead code.

The statement archive keeps last month's 1,200 account records.

A shop's catalog code works out VAT in a legacy module that 40 files import. To move product categories to a new tax service one at a time, starting with books, Ivy added this seam, and start-up code now passes useVat a function that sends books to the new service:

A reviewer comments: "A reassignable module let is a hack.