Ristretto runs your tests.

Usage no npm install needed!

<script type="module">
  import polymerRistretto from 'https://cdn.skypack.dev/@polymer/ristretto';



Ristretto is an extensible test runner.

There are several established test runners in the JavaScript ecosystem (Mocha, Jasmine and Jest to name a few of the more prominent ones). Ristretto was created to address a feature sweet spot left unaddressed by incumbent projects.

Ristretto has the following qualities:

  • Simple, concise, class-based factorization
  • Consumable as modules with browser-compatible path specifiers OOB
  • Batteries included for the most popular testing syntaxes (BDD/TDD)
  • Designed for extensibility (mixins, specialized reporters and a spec-as-data-structure philosophy)
  • Ships with mixins that enhance specs with powerful features (e.g., fixtures and test isolation)
  • Authored in TypeScript
  • No build tooling required to use
  • "JavaScript is the native language, The Web is the native land"

Ristretto is intended to be a single-responsibility detail of a broader, more comprehensive testing regime. In this regard, it is most similar to Mocha in its breadth of capabilities.

Currently, Ristretto only supports direct usage in a web browser with ESM support. Other browsers and platforms (such as Node.js) should work fine with sufficient code transformations applied to this module.


npm install @polymer/ristretto

Writing tests

All tests start with crafting a spec and exporting that spec for consumption elsewhere. Here is an example of a spec written with Ristretto:

// Import the Spec class from Ristretto:
import { Spec } from '../../@polymer/ristretto/lib/spec.js';

// Create a Spec instance that represents our spec:
const spec = new Spec();

// These "describe" and "it" methods work as you would expect when writing
// a test in Mocha, Jasmine or Jest:
const { describe, it } = spec;

// Author your spec as you would in any other test runner:
describe('My spec', () => {
  it('never fails', () => {});

// Export the spec as a module:
export { spec as mySpec };

Running tests

In order to run the tests in your spec, you will need to craft a test suite. A test suite is a collection of specs, and an optional specialized test reporter. Here is a basic example of crafting a test suite and running tests:

// Import the Suite class from Ristretto:
import { Suite } from '../../@polymer/ristretto/lib/suite.js';

// Import the spec we crafted above:
import { mySpec } from './my-spec.js';

// Craft a suite that includes our spec:
const suite = new Suite([ mySpec /*, other, specs, go, here */ ]);

// Run the test suite at your leisure. It will return a promise that resolves
// when all tests have been run:

Reporting results

By default, Ristretto will use a basic console-based reporter called ConsoleReporter. However, it is very easy to craft a custom reporter to suite your needs. Let's write a custom reporter that counts tests and reports how many failed at the end of the test suite run:

// Import the base Reporter class from Ristretto:
import { Reporter } from '../../@polymer/ristretto/lib/reporter.js';

// We export a class that extends the base Reporter class. The reporter has
// a series of test suite life-cycle callbacks that can be optionally
// implemented by child classes to perform specialized reporting. We implement
// a few of them here:
export class CountingReporter extends Reporter {

  onSuiteStart(suite) {
    // Initialize some state when the test suite begins:
    this.totalTestCount = 0;
    this.failedTestCount = 0;

  onTestStart(test, suite) {
    // When a test starts, increment the total test counter:

  onTestEnd(result, test, suite) {
    // If there is an error in the result when a test ends, increment the
    // failed test counter:
    if (result.error) {

  onSuiteEnd(suite) {
    // When the test suite run has completed, announce the total number of
    // tests, and if there were any failed tests announce that number as well:
    console.log(`Total tests: ${this.totalTestCount}`);
    if (this.failedTestCount > 0) {
      console.error(`Failed tests: ${this.failedTestCount}`);
    } else {
      console.log('All tests pass!');

Now that we have a custom reporter, let's use it in a test suite:

// Import the Suite class from Ristretto:
import { Suite } from '../../@polymer/ristretto/lib/suite.js';

// Import the custom reporter we created:
import { CountingReporter } from './counting-reporter.js';

// Craft a suite that includes any specs and our custom reporter:
const suite = new Suite([ /* specs, go, here */ ], new CountingReporter());

// Run the test suite at your leisure. The suite will invoke reporter callbacks
// is testing progresses to completion: