Promise-based IO for JavaScript.
Latest commit aa93b74 Jan 20, 2015 @kriszyp Fix merge conflicts, fixes #53

Promised-IO is a cross-platform package for asynchronous promise-based IO. Promises provide a simple robust mechanism for asynchronicity with separation of concerns by encapsulating eventual completion of an operation with side effect free callback registration separate from call invocation. Promised-IO provides cross-platform file, HTTP, and system interaction with promises for asynchronous operations.

Promised-IO also utilizes "lazy arrays" for progressively completed actions or for streaming of data. Lazy arrays provide all the standard iterative Array methods for receiving callbacks as actions are completed. Lazy arrays are utilized for progressive loading of files and HTTP responses.


Promised-IO can be installed via npm:

npm install promised-io


The promise module provides the primary tools for creating new promises and interacting with promises. The promise API used by promised-io is the Promises/A proposal used by Dojo, jQuery, and other toolkits. Within promised-io, a promise is defined as any object that implements the Promises/A API, that is they provide a then() method that can take a callback. The then() methods definition is:

promise.then(fulfilledHandler, errorHandler);

Promises can originate from a variety of sources, and promised-io provides a constructor, Deferred, to create promises.


when = require("promised-io/promise").when;
when(promiseOrValue, fulfilledHandler, errorHandler);

You can pass a promise to the when() function and the fulfillment and error handlers will be registered for it's completion or you can pass a regular value, and the fulfillment handler will be immediately be called. The when function is a staple of working with promises because it allows you to write code that normalizes interaction with synchronous values and asynchronous promises. If you pass in a promise, a new promise for the result of execution of the callback handler will be returned. If you pass a normal value, the return value will be the value returned from the fulfilledHandler.


deferred = require("promised-io/promise").Deferred(canceler);

The Deferred constructor is the primary mechanism for creating new promises. The Deferred object is a form of a promise with an interface for fulfilling or rejecting the promise. A Deferred object is a means for a producer to resolve a promise and it also provides a promise for consumers that are listening for the resolution of the promise. The basic usage pattern looks like:

var Deferred = require("promised-io/promise").Deferred;
function delay(ms, value){
    // create a new Deferred
    var deferred = new Deferred();
        // fulfill the deferred/promise, all listeners to the promise will be notified, and
        // provided the value as the value of the promise
    }, ms);
    // return the promise that is associated with the Deferred object
    return deferred.promise;

The Deferred can optionally take a canceler function. This function will cause resulting promises to have a cancel() method, and if the cancel() method is called, the Deferred will be canceled and the canceler function will be called.

The Deferred object has the following methods and properties:



This will fulfill the Deferred's promise with the provided value. The fulfillment listeners to the promise will be notified.



This will reject the Deferred's promise with the provided error. The error listeners to the promise will be notified.


This is the promise object associated with the Deferred instance. The promise object will not have any of the Deferred's fulfill or reject methods, and only provides an interface for listening. This can be safely provided to consumers without any chance of being modified.



This will cancel the Deferred.


One of the challenges with working asynchronous code is that there can be times when you wish for some contextual state information to be preserved across multiple asynchronous actions, without having to actually pass the state to each function in the asynchronous chain. Common examples of such contextual state would be tracking the current transaction or the currently logged in user. Such state information could be stored in a singleton (a module property or a global variable), but with asynchronous actions being interleaved, this is unsuitable for tracking state across asynchronous continuations of an action.

The promised-io package's promise module provides a facility for tracking state across asynchronous operations. The promise module tracks the "currentContext" global variable, and whatever value that was in the variable at the time a promise was created will be restored when that promise is fulfilled (or rejected).


group = require("promised-io/promise").all(arrayOfPromises);

The all() function can be passed an array of promises, or multiple promises as individual arguments, and all() will return a new promise that represents the completed values when all the promises have been fulfilled. This allows you to easily run multiple asynchronous actions, and wait for the completion ("join") of all the actions. For example:

group = all(promise1, promise2, promise3);
    var value1 = array[0]; // result of promise1
    var value2 = array[1]; // result of promise2
    var value3 = array[2]; // result of promise3


first = require("promised-io/promise").first(arrayOfPromises);

The first() function can be passed an array of promises, or multiple promises as individual arguments, and first() will return a new promise that represents the completed value when the first promise is fulfilled. This allows you to run multiple asynchronous actions and get the first result. For example:

response = first(requestToMainSite, requestToMirrorSite1, requestToMirrorSite2);
    // response from the first site to respond


result = require("promised-io/promise").seq(arrayOfActionFunctions, startingValue);

The seq() function can be passed an array of functions, and seq() will execute each function in sequence, waiting for the promise returned from each one to complete before executing the next function. Each function will be called with the result of the last function (or the startingValue for the first function).


resultPromise = require("promised-io/promise").whenPromise(valueOrPromise, fulfillmentHandler, errorHandler);

The whenPromise() function behaves exactly like when() except that whenPromise will always return a promise, even if a non-promise value is passed in.


group = require("promised-io/promise").allKeys(hashOfPromises);

Takes a hash of promises and returns a promise that is fulfilled once all the promises in the hash keys are fulfilled.


This module provides promise-based access to the filesystem. The API of the fs module basically follows the Node File System module API. Each of the asynchronous functions in the Node's FS API is reflected with a corresponding function in the fs module that returns a promise (instead of requiring a callback argument in the initial call). For example, where Node has fs.rename(path1, path2, [callback]), with promised-io you would call it:

var fs = require("promised-io/fs");
fs.rename(path1, path2).then(function(){
    // finished renaming

Any callback arguments will be the same minus the error argument:

var fs = require("promised-io/fs");
    // use the result
}, function(error) {
    // Handle errors here instead

One function that does differ from NodeJS's fs module is the open() function.


var file = require("promised-io/fs").open(path, mode);

The open() function differs from simply being a promise-based version of the Node's open() function in that it immediately returns (even though the opening of the file is asynchronous) a file object that be used to read from and write to the file.

To write to the file object, we can write:

promiseForCompletion = file.write(contents, options, encoding);

To close the file object, we can write:

promiseForCompletion = file.close();

We can also use file.writeSync and file.closeSync for the synchronous versions of these functions.

The file object is also a lazy array, which means you can read from the file using standard array methods. To asynchronously read the contents of a file, you can do:

    // called for each chunk of the file until the end of the file is reached.


The lazy-array module provides the functionality for creating and using lazy arrays, which are objects that implement the interface of the standard iterative array methods for accessing streams of data. Array methods can be called and they will be asynchronously executed as data is available. Lazy arrays are powerful way to model asynchronous streams since they can used like other JavaScript arrays.

Typically you don't need to directly use this module, rather other IO modules like the file system (fs) and HTTP (http-client) modules provide lazy arrays that you can interact with. For example, we could search through a file for the string "lazy" and stop reading once we find it using the standard some() method:

    return chunk.toString().indexOf("lazy") > -1;

Lazy arrays include the follow standard array methods, providing access to the data as the stream data becomes available:

  • forEach
  • concat

Additional iterative methods can also access data as it available and as it is requested from the returned lazy array. This means that in order for this function to be excuted, the resulting array should have a forEach (or any of the other standard methods) called on it to trigger the request for data. These methods follow this behavior:

  • filter
  • every
  • some
  • map

And also these standard methods, although these must fully fetch the stream:

  • join
  • sort
  • reverse

Also the following additional methods are available on lazy arrays:

  • toRealArray() - This will fetch all the data and return it as a real JavaScript array.
  • get(index) - This retrieves an element by index.


lazyArray = require("promised-io/lazy-array").LazyArray({
    some: someImplementation,
    length: arrayLength

This function is a constructor for creating your own lazy arrays. With this function, you don't need to implement the entire set of array methods, you can just implement the some() method and provide an array length, if it is known.


first = require("promised-io/lazy-array").first(lazyArray);

This function returns the first element in a lazy array.


last = require("promised-io/lazy-array").last(lazyArray);

This function returns the last element in a lazy array.


item = require("promised-io/lazy-array").get(index);

This function returns an element by index from a lazy array.


This module provides convenient promise-based access to making HTTP requests.

Promised-IO is part of the Persevere project, and therefore is licensed under the AFL or BSD license. The Persevere project is administered under the Dojo foundation, and all contributions require a Dojo CLA.