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Promises/A+ logo

Promises/A+ Compliance Test Suite

This suite tests compliance of a promise implementation with the Promises/A+ specification.

Passing the tests in this repo means that you have a Promises/A+ compliant implementation of the then() method, and you can display the Promises/A+ logo in your README. You can also send a pull request to have your implementation listed on the implementations page.

How To Run

The tests can run in either a Node.js environment or, if you set things up correctly, in the browser.


In order to test your promise library, you must expose a very minimal adapter interface. These are written as Node.js modules with a few well-known exports:

  • resolved(value): creates a promise that is resolved with value.
  • rejected(reason): creates a promise that is already rejected with reason.
  • deferred(): creates an object consisting of { promise, resolve, reject }:
    • promise is a promise that is currently in the pending state.
    • resolve(value) resolves the promise with value.
    • reject(reason) moves the promise from the pending state to the rejected state, with rejection reason reason.

The resolved and rejected exports are actually optional, and will be automatically created by the test runner using deferred if they are not present. But, if your promise library has the capability to create already-resolved or already-rejected promises, then you should include these exports, so that the test runner can provide you with better code coverage and uncover any bugs in those methods.

Note that the tests will never pass a promise or a thenable as a resolution. That means that we never use the promise- or thenable-accepting forms of the resolve operation directly, and instead only use the direct fulfillment operation, since fulfill and resolve are equivalent when not given a thenable.

Finally, note that none of these functions, including deferred().resolve and deferred().reject, should throw exceptions. The tests are not structured to deal with that, and if your implementation has the potential to throw exceptions—e.g., perhaps it throws when trying to resolve an already-resolved promise—you should wrap direct calls to your implementation in try/catch when writing the adapter.

From the CLI

This package comes with a command-line interface that can be used either by installing it globally with npm install promises-aplus-tests -g or by including it in your package.json's devDependencies and using npm's scripts feature. In the latter case, your setup might look something like

    "devDependencies": {
        "promises-aplus-tests": "*"
    "scripts": {
        "test": "run-my-own-tests && promises-aplus-tests test/my-adapter"

The CLI takes as its first argument the filename of your adapter file, relative to the current working directory. It tries to pass through any subsequent options to Mocha, so you can use e.g. --reporter spec or --grep 2.2.4.


The main export of this package is a function that allows you to run the tests against an adapter:

var promisesAplusTests = require("promises-aplus-tests");

promisesAplusTests(adapter, function (err) {
    // All done; output is in the console. Or check `err` for number of failures.

You can also pass any Mocha options as the second parameter, e.g.

promisesAplusTests(adapter, { reporter: "dot" }, function (err) {
  // As before.

Within an Existing Mocha Test Suite

If you already have a Mocha test suite and want to include these tests in it, you can do:

describe("Promises/A+ Tests", function () {

This also works in the browser, if you have your Mocha tests running there, as long as you use browserify.