A lightweight implementation of CommonJS Promises/A for PHP.

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Table of Contents

  1. Introduction
  2. Concepts
  3. API
  4. Examples
  5. Credits
  6. License


React/Promise is a library implementing CommonJS Promises/A for PHP.

It also provides several other useful promise-related concepts, such as joining multiple promises and mapping and reducing collections of promises.

If you've never heard about promises before, read this first.



A Deferred represents a computation or unit of work that may not have completed yet. Typically (but not always), that computation will be something that executes asynchronously and completes at some point in the future.


While a deferred represents the computation itself, a Promise represents the result of that computation. Thus, each deferred has a promise that acts as a placeholder for its actual result.



A deferred represents an operation whose resolution is pending. It has separate promise and resolver parts.

$deferred = new React\Promise\Deferred();

$promise = $deferred->promise();

$deferred->resolve(mixed $value = null);
$deferred->reject(mixed $reason = null);
$deferred->notify(mixed $update = null);

The promise method returns the promise of the deferred.

The resolve and reject methods control the state of the deferred.

The notify method is for progress notification.

The constructor of the Deferred accepts an optional $canceller argument. See Promise for more information.


$promise = $deferred->promise();

Returns the promise of the deferred, which you can hand out to others while keeping the authority to modify its state to yourself.


$deferred->resolve(mixed $value = null);

Resolves the promise returned by promise(). All consumers are notified by having $onFulfilled (which they registered via $promise->then()) called with $value.

If $value itself is a promise, the promise will transition to the state of this promise once it is resolved.


$deferred->reject(mixed $reason = null);

Rejects the promise returned by promise(), signalling that the deferred's computation failed. All consumers are notified by having $onRejected (which they registered via $promise->then()) called with $reason.

If $reason itself is a promise, the promise will be rejected with the outcome of this promise regardless whether it fulfills or rejects.


$deferred->notify(mixed $update = null);

Triggers progress notifications, to indicate to consumers that the computation is making progress toward its result.

All consumers are notified by having $onProgress (which they registered via $promise->then()) called with $update.


The promise interface provides the common interface for all promise implementations.

A promise represents an eventual outcome, which is either fulfillment (success) and an associated value, or rejection (failure) and an associated reason.

Once in the fulfilled or rejected state, a promise becomes immutable. Neither its state nor its result (or error) can be modified.



$transformedPromise = $promise->then(callable $onFulfilled = null, callable $onRejected = null, callable $onProgress = null);

Transforms a promise's value by applying a function to the promise's fulfillment or rejection value. Returns a new promise for the transformed result.

The then() method registers new fulfilled, rejection and progress handlers with a promise (all parameters are optional):

It returns a new promise that will fulfill with the return value of either $onFulfilled or $onRejected, whichever is called, or will reject with the thrown exception if either throws.

A promise makes the following guarantees about handlers registered in the same call to then():

  1. Only one of $onFulfilled or $onRejected will be called, never both.
  2. $onFulfilled and $onRejected will never be called more than once.
  3. $onProgress may be called multiple times.

See also


The ExtendedPromiseInterface extends the PromiseInterface with useful shortcut and utility methods which are not part of the Promises/A specification.



$promise->done(callable $onFulfilled = null, callable $onRejected = null, callable $onProgress = null);

Consumes the promise's ultimate value if the promise fulfills, or handles the ultimate error.

It will cause a fatal error if either $onFulfilled or $onRejected throw or return a rejected promise.

Since the purpose of done() is consumption rather than transformation, done() always returns null.

See also


$promise->otherwise(callable $onRejected);

Registers a rejection handler for promise. It is a shortcut for:

$promise->then(null, $onRejected);

Additionally, you can type hint the $reason argument of $onRejected to catch only specific errors.

    ->otherwise(function (\RuntimeException $reason) {
        // Only catch \RuntimeException instances
        // All other types of errors will propagate automatically
    ->otherwise(function ($reason) {
        // Catch other errors


$newPromise = $promise->always(callable $onFulfilledOrRejected);

Allows you to execute "cleanup" type tasks in a promise chain.

It arranges for $onFulfilledOrRejected to be called, with no arguments, when the promise is either fulfilled or rejected.

always() behaves similarly to the synchronous finally statement. When combined with otherwise(), always() allows you to write code that is similar to the familiar synchronous catch/finally pair.

Consider the following synchronous code:

try {
  return doSomething();
} catch(\Exception $e) {
    return handleError($e);
} finally {

Similar asynchronous code (with doSomething() that returns a promise) can be written:

return doSomething()


$promise->progress(callable $onProgress);

Registers a handler for progress updates from promise. It is a shortcut for:

$promise->then(null, null, $onProgress);


A cancellable promise provides a mechanism for consumers to notify the creator of the promise that they are not longer interested in the result of an operation.



The cancel() method notifies the creator of the promise that there is no further interest in the results of the operation.

Once a promise is settled (either fulfilled or rejected), calling cancel() on a promise has no effect.



Creates a promise whose state is controlled by the functions passed to $resolver.

$resolver = function (callable $resolve, callable $reject, callable $notify) {
    // Do some work, possibly asynchronously, and then
    // resolve or reject. You can notify of progress events
    // along the way if you want/need.

    // or $resolve($anotherPromise);
    // or $reject($nastyError);
    // or $notify($progressNotification);

$canceller = function (callable $resolve, callable $reject, callable $progress) {
    // Cancel/abort any running operations like network connections, streams etc.

    $reject(new \Exception('Promise cancelled'));

$promise = new React\Promise\Promise($resolver, $canceller);

The promise constructor receives a resolver function and an optional canceller function which both will be called with 3 arguments:

If the resolver or canceller throw an exception, the promise will be rejected with that thrown exception as the rejection reason.

The resolver function will be called immediately, the canceller function only once all consumers called the cancel() method of the promise.


Creates a already fulfilled promise.

$promise = React\Promise\FulfilledPromise($value);

Note, that $value cannot be a promise. It's recommended to use resolve() for creating resolved promises.


Creates a already rejected promise.

$promise = React\Promise\RejectedPromise($reason);

Note, that $reason cannot be a promise. It's recommended to use reject() for creating rejected promises.


Creates a promise which will be lazily initialized by $factory once a consumer calls the then() method.

$factory = function () {
    $deferred = new React\Promise\Deferred();

    // Do some heavy stuff here and resolve the deferred once completed

    return $deferred->promise();

$promise = React\Promise\LazyPromise($factory);

// $factory will only be executed once we call then()
$promise->then(function ($value) {


Useful functions for creating, joining, mapping and reducing collections of promises.

All functions working on promise collections (like all(), race(), some() etc.) support cancellation. This means, if you call cancel() on the returned promise, all promises in the collection are cancelled. If the collection itself is a promise which resolves to an array, this promise is also cancelled.


$promise = React\Promise\resolve(mixed $promiseOrValue);

Creates a promise for the supplied $promiseOrValue.

If $promiseOrValue is a value, it will be the resolution value of the returned promise.

If $promiseOrValue is a thenable (any object that provides a then() method), a trusted promise that follows the state of the thenable is returned.

If $promiseOrValue is a promise, it will be returned as is.

Note: The promise returned is always a promise implementing ExtendedPromiseInterface. If you pass in a custom promise which only implements PromiseInterface, this promise will be assimilated to a extended promise following $promiseOrValue.


$promise = React\Promise\reject(mixed $promiseOrValue);

Creates a rejected promise for the supplied $promiseOrValue.

If $promiseOrValue is a value, it will be the rejection value of the returned promise.

If $promiseOrValue is a promise, its completion value will be the rejected value of the returned promise.

This can be useful in situations where you need to reject a promise without throwing an exception. For example, it allows you to propagate a rejection with the value of another promise.


$promise = React\Promise\all(array|React\Promise\PromiseInterface $promisesOrValues);

Returns a promise that will resolve only once all the items in $promisesOrValues have resolved. The resolution value of the returned promise will be an array containing the resolution values of each of the items in $promisesOrValues.


$promise = React\Promise\race(array|React\Promise\PromiseInterface $promisesOrValues);

Initiates a competitive race that allows one winner. Returns a promise which is resolved in the same way the first settled promise resolves.


$promise = React\Promise\any(array|React\Promise\PromiseInterface $promisesOrValues);

Returns a promise that will resolve when any one of the items in $promisesOrValues resolves. The resolution value of the returned promise will be the resolution value of the triggering item.

The returned promise will only reject if all items in $promisesOrValues are rejected. The rejection value will be an array of all rejection reasons.

The returned promise will also reject with a React\Promise\Exception\LengthException if $promisesOrValues contains 0 items.


$promise = React\Promise\some(array|React\Promise\PromiseInterface $promisesOrValues, integer $howMany);

Returns a promise that will resolve when $howMany of the supplied items in $promisesOrValues resolve. The resolution value of the returned promise will be an array of length $howMany containing the resolution values of the triggering items.

The returned promise will reject if it becomes impossible for $howMany items to resolve (that is, when (count($promisesOrValues) - $howMany) + 1 items reject). The rejection value will be an array of (count($promisesOrValues) - $howMany) + 1 rejection reasons.

The returned promise will also reject with a React\Promise\Exception\LengthException if $promisesOrValues contains less items than $howMany.


$promise = React\Promise\map(array|React\Promise\PromiseInterface $promisesOrValues, callable $mapFunc);

Traditional map function, similar to array_map(), but allows input to contain promises and/or values, and $mapFunc may return either a value or a promise.

The map function receives each item as argument, where item is a fully resolved value of a promise or value in $promisesOrValues.


$promise = React\Promise\reduce(array|React\Promise\PromiseInterface $promisesOrValues, callable $reduceFunc , $initialValue = null);

Traditional reduce function, similar to array_reduce(), but input may contain promises and/or values, and $reduceFunc may return either a value or a promise, and $initialValue may be a promise or a value for the starting value.


The React\Promise\PromisorInterface provides a common interface for objects that provide a promise. React\Promise\Deferred implements it, but since it is part of the public API anyone can implement it.


How to use Deferred

function getAwesomeResultPromise()
    $deferred = new React\Promise\Deferred();

    // Execute a Node.js-style function using the callback pattern
    computeAwesomeResultAsynchronously(function ($error, $result) use ($deferred) {
        if ($error) {
        } else {

    // Return the promise
    return $deferred->promise();

        function ($value) {
            // Deferred resolved, do something with $value
        function ($reason) {
            // Deferred rejected, do something with $reason
        function ($update) {
            // Progress notification triggered, do something with $update

How promise forwarding works

A few simple examples to show how the mechanics of Promises/A forwarding works. These examples are contrived, of course, and in real usage, promise chains will typically be spread across several function calls, or even several levels of your application architecture.

Resolution forwarding

Resolved promises forward resolution values to the next promise. The first promise, $deferred->promise(), will resolve with the value passed to $deferred->resolve() below.

Each call to then() returns a new promise that will resolve with the return value of the previous handler. This creates a promise "pipeline".

$deferred = new React\Promise\Deferred();

    ->then(function ($x) {
        // $x will be the value passed to $deferred->resolve() below
        // and returns a *new promise* for $x + 1
        return $x + 1;
    ->then(function ($x) {
        // $x === 2
        // This handler receives the return value of the
        // previous handler.
        return $x + 1;
    ->then(function ($x) {
        // $x === 3
        // This handler receives the return value of the
        // previous handler.
        return $x + 1;
    ->then(function ($x) {
        // $x === 4
        // This handler receives the return value of the
        // previous handler.
        echo 'Resolve ' . $x;

$deferred->resolve(1); // Prints "Resolve 4"

Rejection forwarding

Rejected promises behave similarly, and also work similarly to try/catch: When you catch an exception, you must rethrow for it to propagate.

Similarly, when you handle a rejected promise, to propagate the rejection, "rethrow" it by either returning a rejected promise, or actually throwing (since promise translates thrown exceptions into rejections)

$deferred = new React\Promise\Deferred();

    ->then(function ($x) {
        throw new \Exception($x + 1);
    ->otherwise(function (\Exception $x) {
        // Propagate the rejection
        throw $x;
    ->otherwise(function (\Exception $x) {
        // Can also propagate by returning another rejection
        return React\Promise\reject(
            new \Exception($x->getMessage() + 1)
    ->otherwise(function ($x) {
        echo 'Reject ' . $x->getMessage(); // 3

$deferred->resolve(1);  // Prints "Reject 3"

Mixed resolution and rejection forwarding

Just like try/catch, you can choose to propagate or not. Mixing resolutions and rejections will still forward handler results in a predictable way.

$deferred = new React\Promise\Deferred();

    ->then(function ($x) {
        return $x + 1;
    ->then(function ($x) {
        throw new \Exception($x + 1);
    ->otherwise(function (\Exception $x) {
        // Handle the rejection, and don't propagate.
        // This is like catch without a rethrow
        return $x->getMessage() + 1;
    ->then(function ($x) {
        echo 'Mixed ' . $x; // 4

$deferred->resolve(1);  // Prints "Mixed 4"

Progress event forwarding

In the same way as resolution and rejection handlers, your progress handler MUST return a progress event to be propagated to the next link in the chain. If you return nothing, null will be propagated.

Also in the same way as resolutions and rejections, if you don't register a progress handler, the update will be propagated through.

If your progress handler throws an exception, the exception will be propagated to the next link in the chain. The best thing to do is to ensure your progress handlers do not throw exceptions.

This gives you the opportunity to transform progress events at each step in the chain so that they are meaningful to the next step. It also allows you to choose not to transform them, and simply let them propagate untransformed, by not registering a progress handler.

$deferred = new React\Promise\Deferred();

    ->progress(function ($update) {
        return $update + 1;
    ->progress(function ($update) {
        echo 'Progress ' . $update; // 2

$deferred->notify(1);  // Prints "Progress 2"

done() vs. then()

The golden rule is:

Either return your promise, or call done() on it.

At a first glance, then() and done() seem very similar. However, there are important distinctions.

The intent of then() is to transform a promise's value and to pass or return a new promise for the transformed value along to other parts of your code.

The intent of done() is to consume a promise's value, transferring responsibility for the value to your code.

In addition to transforming a value, then() allows you to recover from, or propagate intermediate errors. Any errors that are not handled will be caught by the promise machinery and used to reject the promise returned by then().

Calling done() transfers all responsibility for errors to your code. If an error (either a thrown exception or returned rejection) escapes the $onFulfilled or $onRejected callbacks you provide to done, it will be rethrown in an uncatchable way causing a fatal error.

function getJsonResult()
    return queryApi()
            // Transform API results to an object
            function ($jsonResultString) {
                return json_decode($jsonResultString);
            // Transform API errors to an exception
            function ($jsonErrorString) {
                $object = json_decode($jsonErrorString);
                throw new ApiErrorException($object->errorMessage);

// Here we provide no rejection handler. If the promise returned has been
// rejected, the ApiErrorException will be thrown
        // Consume transformed object
        function ($jsonResultObject) {
            // Do something with $jsonResultObject

// Here we provide a rejection handler which will either throw while debugging
// or log the exception
        function ($jsonResultObject) {
            // Do something with $jsonResultObject
        function (ApiErrorException $exception) {
            if (isDebug()) {
                throw $exception;
            } else {

Note that if a rejection value is not an instance of \Exception, it will be wrapped in an exception of the type React\Promise\UnhandledRejectionException.

You can get the original rejection reason by calling $exception->getReason().


React/Promise is a port of when.js by Brian Cavalier.

Also, large parts of the documentation have been ported from the when.js Wiki and the API docs.


React/Promise is released under the MIT license.