Transactional tests with Mocha.js

This week, while working on the email change scenario on my side project, I realized that my tests are not transactional, which meant that their changes to the DB could persist across test runs and so tests could influence each other. Hm… I thought I fixed this before.

The whole idea of transactional tests comes for me from RSpec, and I was surprised when I was unable to google anything about it in the context of Mocha.js.

So, I’m using Mocha.js for my test suite, and mysql NPM module for my persistence, and here is my initial take on this, about a month ago:

connectionPool.config.connectionLimit = 1;
connectionPool.on("connection", connection => {
  beforeEach(() => {
    connection.beginTransaction(e => {
      if (e) {
        throw e;

  afterEach(async () => connection.rollback());

The concept is quite straight forward: have top-level beforeEach and afterEach hooks that begin a DB transaction before every test, and roll it back after, correspondingly. Although this code seemed reasonable at the time, the catch here is that the connection event only happens when the first query is run, and so, these hooks are set long after they should have been. I guess that my tests at that time didn’t have a scenario that would make this issue clear, and so it slipped through.

Now, here is the new setup, based on the new understanding:


async function makeTestsTransactional() {
  connectionPool.on("connection", (connection) => {
    beforeEach(() => {
      connection.beginTransaction((e) => expect(e);

    afterEach(() => {

  await q(`SELECT 'Trigger "connection" above to wrap tests in a transaction';`);

The code is pretty much the same as before, with the exception of the last line, where I run a fake SQL query right away, just to trigger the connection event early and have the global hooks set as soon as possible.

As a bonus, I also added a verification after all the tests are run, specifically I expect all my application tables to be empty:


async function expectEmptyTables() {
  const tablesToExclude = ["migrations", "sessions"];

  const tableNames = (await q(`SHOW TABLES`))
    .map(({Tables_in_repetitor_test: tableName}) => tableName as string)
    .filter((x) => !tablesToExclude.includes(x));

  const getRowCount = async (tableName: string) => (await q(`SELECT * FROM ${tableName}`)).length;
  const getTableTuple = async (tableName: string) => [tableName, await getRowCount(tableName)] as [string, number];

  const rowCounts = Object.fromEntries(await Promise.all(;
  const expectedRowCounts = Object.fromEntries( => [n, 0]));

  expect(rowCounts, "some tables have rows after running tests").to.deep.equal(expectedRowCounts);

It’s quite dense, but not unintelligible: I essentially list and query all the application tables, then expect that they each have 0 rows.

OK, that’s better. Happy testing!