Cart specs started failing at random once we ran them in parallel

Problem Description

features/cart.feature and its step code have been stable for months. Last week we turned on parallel execution to get the suite under five minutes, and since then roughly one run in three fails - never the same scenario twice.

The failures come in two shapes. Sometimes it is TypeError: Cannot read properties of undefined (reading 'applyPromo'). Sometimes a total is wrong by the exact amount of another scenario's cart, which is worse, because the numbers look plausible enough that a reviewer waves them through.

Drop back to a single process and the file passes every time. Nobody has changed src/cart.js.

The two scenarios that carry no starting cart of their own were written when the file always ran top to bottom, and they read whatever the previous scenario left behind.

Output Specification

Rework the step code (and the feature only as far as point 3 requires) so the suite is correct at any level of parallelism:

1. No value produced while one scenario runs may still be reachable when the next one starts. Each scenario must get its own.
2. Leave the parallel setting in cucumber.js as it is - the point is to make the suite correct under it, not to avoid it.
3. Every scenario must declare the cart it needs. Adding the missing declaration to the two scenarios that lack one is in scope; rewording the other lines is not.
4. src/cart.js is production code and must not change.
5. All five behaviours stay covered.

Input Files

Extract the following files before beginning.

=============== FILE: features/cart.feature =============== Feature: Shopping cart totals

Scenario: Two of the same item Given an empty cart And the cart contains 2 of "BOOK-001" at $12.50 Then the cart holds 2 items And the total is $25.00

Scenario: A promo applies to the whole cart Given an empty cart And the cart contains 1 of "BOOK-001" at $12.50 And the cart contains 3 of "MUG-014" at $7.00 When I apply the promo code "WELCOME10" Then the total is $30.15

Scenario: An unknown promo is rejected When I apply the promo code "NOTREAL" Then the promo is rejected And the total is $33.50

Scenario: Four of one item Given an empty cart And the cart contains 4 of "MUG-014" at $7.00 Then the cart holds 4 items And the total is $28.00

Scenario: The promo survives a repeated item When I apply the promo code "WELCOME10" Then the total is $25.20

=============== FILE: features/step_definitions/cart.steps.js =============== const assert = require('node:assert'); const { Given, When, Then } = require('@cucumber/cucumber'); const { Cart } = require('../../src/cart');

let cart; let promoAccepted;

Given('an empty cart', () => { cart = new Cart(); });

Given('the cart contains {int} of {string} at ${float}', (qty, sku, price) => { cart.add(sku, qty, price); });

When('I apply the promo code {string}', (code) => { promoAccepted = cart.applyPromo(code); });

Then('the cart holds {int} items', (expected) => { assert.strictEqual(cart.itemCount(), expected); });

Then('the total is ${float}', (expected) => { assert.strictEqual(cart.total(), expected); });

Then('the promo is rejected', () => { assert.strictEqual(promoAccepted, false); });

=============== FILE: src/cart.js =============== class Cart { constructor() { this.lines = []; this.discount = 0; }

add(sku, qty, price) { this.lines.push({ sku, qty, price }); }

itemCount() { return this.lines.reduce((n, line) => n + line.qty, 0); }

subtotal() { return this.lines.reduce((n, line) => n + line.qty * line.price, 0); }

applyPromo(code) { this.discount = code === 'WELCOME10' ? 0.1 : 0; return this.discount > 0; }

total() { return Number((this.subtotal() * (1 - this.discount)).toFixed(2)); } }

module.exports = { Cart };

=============== FILE: cucumber.js =============== module.exports = { default: { require: ['features/step_definitions//*.js', 'features/support//*.js'], format: ['progress'], parallel: 2, }, };

=============== FILE: package.json =============== { "name": "storefront-specs", "private": true, "scripts": { "bdd": "cucumber-js" }, "devDependencies": { "@cucumber/cucumber": "^10.9.0" } }