Proof of concept helper for snapshot testing in Mocha

src __snapshots__ integration test.snapshot.js

exports['mocha snapshot checker always works for strings'] = `
foo
hello
world
`.slice(1, -1); // remove leading & trailing newlines

exports['mocha snapshot checker always works for objects'] = {
  key: 'valuex',
  number: 1,
};

src __tests__ integration test.js

const createSnapshotChecker = require('../../snapshot');
const path = require('path');

describe('mocha snapshot checker', () => {
  let expectSnapshot;
  beforeEach(function setupSnapshots() {
    expectSnapshot = createSnapshotChecker(
      path.resolve(__dirname, '../../__snapshots__'),
      this,
    );
  });

  context('always', () => {
    it('works for strings', function() {
      expectSnapshot('foo\nbar\nbaz');
    });

    it('works for objects', function() {
      expectSnapshot({key: 'value', number: 1});
    });
  });
});

src snapshot.js

const assert = require('assert');
const path = require('path');

module.exports = function createSnapshotChecker(snapshotDir, beforeEachCtx) {
  const currentTest = beforeEachCtx.currentTest;

  const title = buildTitle(currentTest);
  const snapshotFile = buildSnapshotFile(snapshotDir, currentTest);
  const data = loadSnapshots(snapshotFile);
  let counter = 0;

  return function expectSnapshot(actual) {
    const key = counter ? `${title}-${counter}` : title;
    counter++;

    if (!(key in data)) {
      throw new Error(`No snapshot found for ${JSON.stringify(key)}.`);
    }

    assert.deepStrictEqual(actual, data[key]);
  };
}

function buildTitle(ctx) {
  let result = ctx.title;
  for (;;) {
    if (!ctx.parent) break;
    ctx = ctx.parent;
    if (ctx.title) {
      result = ctx.title + ' ' + result;
    }
  }
  return result;
}

function buildSnapshotFile(snapshotDir, currentTest) {
  const parsed = path.parse(currentTest.file);
  const parts = path.normalize(parsed.dir).split(path.sep);

  const ix = parts.findIndex(p => p === 'test' || p === '__tests__');
  if (ix < 0) {
    throw new Error(
      'Snapshot checker requires test files in `test` or `__tests__`'
    );
  }

  // Remove everything from start up to (including) `test` or `__tests__`
  parts.splice(0, ix+1);

  return path.join(snapshotDir, ...parts, parsed.name + '.snapshot.js');
}

function loadSnapshots(snapshotFile) {
  try {
    return require(snapshotFile);
  } catch (err) {
    console.log(err.code);
    if (err.code === 'MODULE_NOT_FOUND') {
      console.warn(
        'Warning: snapshot file %j was not found.',
        path.relative(process.cwd(), snapshotFile),
      );
      return {};
    }
    throw err;
  }
}