Promish

     SH
      I
P R O M

The Promish module creates a wrapper around the EcmaScript 6 Promise class (and others). It adds some of the useful features found in many of the other popular promise libraries such as Q and Bluebird. It is designed to be interchangeable with the ES6 Promise as its interface is a superset of the Promise class.

I have strived to keep this library as small as practicable while offering as much functionality as possible. As it stands, the minified browserified bundle is less than 14k!

Installation

npm install promish

New Features!

• Browserification
  • The Promish class has been reworked to allow the base-promise class it extends to be injected so that a browser friendly promise can be used for the browserify build. The implementation I am using for this is es6-promise.
  • The standard Promish implementation (via require('promish') has not been affected and will still deliver a Promish class that extends the native Promise.
  • See Browserification for details.
• map
  • map takes an array of values or promises and calls a supplied callback function on each resolved value finally resolving in an array of values
• reduce
  • reduce takes an array of values or promises and calls a supplied callback function on each resolved value in a sequential fashion resolving to a single value

Contents

• Interface
  • Include
  • Instantiation
  • Then
  • Catch
  • Finally
  • Promishification
    • Apply
    • Call
    • Post
    • Invoke
    • Promisify
    • Promisify All
    • Method
  • Race
  • Some
  • Any
  • Spread
  • Map
  • Reduce
• Browserification
• Known Issues
• Release History

Interface

Include

var Promish = require('promish');

Instantiation

Typical use - construct with handler function

var promise = new Promish(function(resolve, reject) {
  // do something async
});

3rd Party Wrapper Mode

var promise = new Promish(Q());
 
var promise = new Promish(new Promise( ... ));

Value Wrapper Mode

// If the constructor value is not a function, a thenable or an Error,
// assume it should be a resolve value.
var promise = new Promish('Resolve Value');
 
// To explicitly signal resolve, use Promish.resolve
var promise = Promish.resolve('Resolve Value');

Error Wrapper Mode

// If the constructor value is an Error type, it will be interpreted as rejection
var promise = new Promish(new Error('This promise is rejected'));
 
// To explicitly signal something is rejection use Promish.reject
var promise = Promish.reject('This is not an error object, but reject with it anyway')

Then

// typical use
promise
  .then(function(value) {
    // something async has completed with a value
    // here you can return a resolve value,
    // return a new Promish or throw an error (handled as rejection)
  });
  
// with onRejected...
promise.then(
  function(value) {
  },
  function(error) {
  });

Catch

The catch function takes a catch handler that will be called when the promise state is rejected and is a more elegant way to handle errors than using the second then argument.

// catch all
promise
  .catch(function(error) {
    // Something async has failed with an error.
    // Just like with then(), you can return a resolve value,
    // return a new Promish or throw a new error (handled as rejection)
    // You can also 'rethrow' the error by returning a new Promish
    // with the error
  });

Promishes also support Error type matching

new Promish(function(resolve, reject) {
    resolve(JSON.parse(text));
  })
  .then(function(json) { ... })
  .catch(SyntaxError, function(error) {
    // only called if error is instanceof SyntaxError
  })
  .catch(function(error) {
    // will be called for other types of error
  });

And also support user supplied error match functions

function isFooString(value) {
  return ((typeof value) === 'string') && (value.indexOf('foo') >= 0);
}
 
promise
  .then(function(value) { ... })
  .catch(isFooString, function(error) {
    // error is a string and contains 'foo'
  })
  .catch(function(error) {
    // called if the not a foo string
  });

Finally

A finally handler will be called no matter what state the promise chain gets into. There are no arguments provided to the finally handler and the downstream promise state will typically reflect the state of the promise before the finally handler is called. If the finally handler returns a promise, finally will wait for the promise to resolve before propagating the incoming promise value. If the finally handler's promise is rejected, the new rejected state will override the incoming promise state and the new state will take on the new rejection state of the finally handler's promise. This will also be the case if the finally handler throws an exception.

// catch all
promise
  .then(function(value) { ... })
  .catch(function(error) { ... })
  .finally(function() {
    // clean stuff up
  })

Delay

Pause for a number of milliseconds and then continue. The resolve value will be preserved. If the promish state is rejected, delay will not delay and will preserve the rejection error

getAPromish()
  .delay(1000)
  .then(function(value) {
    // continue 1 second later
  })
  .catch(function(error) {
    // there was an error, process immediately
  });

Defer

For compatability with the old Promise.defer() pattern...

function readAFile(filename) {
  var deferred = Promish.defer();
  
  fs.readFile(filename, function(error, data) {
    if (error) {
      deferred.reject(error);
    } else {
      deferred.resolve(data);
    }
  });
  
  return deferred.promise;
}

Promisification Calls

The majority of the old asynchronous Node methods follow a basic pattern where the last argument in a function is a callback function and the first argument of that callback function is used to signal errors - if the error argument is truthy, then the call failed and the value of the error will indicate why, otherwise the call succeeded.

Promisification involves converting the async pattern into promises - either on the fly or by wrapping functions, methods or even whole objects...

Apply

// Note: Promish.nfapply alias included for Q compatability
Promish.apply(fs.readFile, [filename])
  .then(function(data) {
    // oooh data!
  })
  .catch(function(error) {
    // handle failure
  });

Call

// Note: Promish.nfcall alias included for Q compatability
Promish.call(fs.readFile, filename)
  .then(function(data) {
    // oooh data!
  })
  .catch(function(error) {
    // handle failure
  });

Post

// call method of target with arguments inline
// Note: Promish.npost alias
Promish.invoke(target, value1, value2)
  .then(function(value) { ... });  

Invoke

// invoke method of target with array of arguments
// Note: Promish.ninvoke alias
Promish.invoke(target, [value1, value2])
  .then(function(value) { ... });  

Promisify

Convert a function from async to promise for future use.

var readFile = Promish.promisify(fs.readFile);
 
readFile(filename)
  .then(function(data) { ... })

Promisify All

Promisify all the async methods of an object.

There are two modes supported:

• Proxy Mode (default)
  • Creates a separate object that contains promisified methods for each method of the target object. The methods typically have the same name
  • Note: ES6 Proxies eagerly awaited here!
• In-Place Mode
  • Adds promisified methods to the object, typically with a suffix to avoid colliding with the actual methods.

 
// Proxy mode:
var fs = Promish.promisifyAll(require('fs'));
fs.readFile(filename)
  .then(function(data) { ... });
 
// In-Place Mode
var fs = Promish.promisifyAll(require('fs'), { inPlace: true, suffix: 'Async' });
fs.readFileAsync(filename)
  .then(function(data) { ... });
 

Method

Wrap a synchronous function or method so that it always returns a promise

var myFunc = Promish.method(function(value) {
  // can throw
  if (!value) throw new Error('Not zero!');
  
  // can return value
  if (value > 0) return value;
  
  // can return promish()
  return Promish.resolve(value);
});
 
myFunc(1234)
  .then(function(value) {
    // ...
  });
 
// also works as member functions
MyClass.prototype.func = Promish.method(function(value) {
  // this is what you think it is
  return this.value = value;
});
 
new MyClass(7).func
  .then(function(value) {
    // ...
  });
 

All

Promish wraps the native implementation of all.

Promish.all([getPromise1(), getPromise2()])
  .then(function(values) { ... });

Race

Promish wraps the native implementation of race.

Promish.race([promise1, promise2])
  .then(function(value) {
    // first promise to finish was a success
  })
  .catch(function(error) {
    // first promise to finish failed
  });
  

Some

Resolve on first N successful promises or reject with array of errors.

Promish.some([promise1, promise2, promise3], 2)
  .then(function(values) {
    // first 2 successful promises...
  })
  .catch(function(errors) {
    // at least 2 promises failed
  });
  

Any

Resolve on first successful promise or reject with array of errors.

Promish.any([promise1, promise2])
  .then(function(value) {
    // first successful promise...
  })
  .catch(function(errors) {
    // all promises failed
  });
  

Spread

Convert a resolve value array into arguments

Promish.all([getPromish1(), getPromish2(), getPromish3()])
  .spread(function(a,b,c) {
    // a === value from getPromish1
    // b === value from getPromish2
    // c === value from getPromish3
  });

Spread will also convert an array of promises into their resolved values

new Promish(function(resolve) {
    resolve([getPromish1(), getPromish2(), getPromish3()])
  })
  .spread(function(a,b,c) {
    // a === value from getPromish1
    // b === value from getPromish2
    // c === value from getPromish3
  });

Map

Process an array of values or promises using supplied callback and resolving with an array of processed values.

function processMapValues(value) {
    return value * 2;
}
// static version
Promish.map([getValue(), getPromise()], processMapValues)
    .spread(function(a,b) {
        // a and b are resolved values
    });
    
// inline version
Promish.resolve([getValue(), getPromise()])
    .map(processMapValues)
    .spread(function(a,b) {
        // a and b are resolved values
    });

Reduce

Process an array of values or promises using supplied callback and resolving with a single accumulated values. The callback is called with arguments of accumulator and resolved value and returns a value or promise which will be resolved to become the next accuumulator value. For further reading on reduce, please consult documentation for Array reduce();

function processReduceValues(total, value) {
    return total + value;
}
 
// static version
Promish.reduce([getValue(), getPromise()], processReduceValues, 0)
    .then(function(total) {
        //  total will be the sum of all resolved values
    });
    
// inline version
Promish.resolve([getValue(), getPromise()])
    .reduce(processReduceValues)
    .then(function(total) {
        //  total will be the sum of all resolved values
    });
    

Browserification

Promish is now also built for browserification both as a standalone bundle and as a module suitable for inclusion into your own browserify build.

dist/promish-bundle

A browserified bundle is included as dist/promish-bundle.js (or dist/promish-bundle.min.js). The bundle uses (and includes) the es6-promise module in order to produce a browser friendly bundle.

dist/promish-node

For node projects that still require older versions of JavaScript (or for including in a different browser bundle), a node friendly module has also been included. To use, add the following code:

var Promish = require('promish/dist/promish-node');

Note that promish-node makes use of ES6 features like Array.from that are not covered by the babel transpile. If you need to use promish in a completely ES2015 environment you will need to include some kind of compatable polyfill:

// polyfill ES6 features
require('babel-polyfill');

Known Issues

• TBD

Release History

Version	Changes
0.0.1	Initial Version
0.0.2	Promish.delay() Promish.defer()
0.0.3	Promish.delay() Promish.defer() Promish.spread()
0.0.4	Promish.apply() Promish.call()
0.0.5	Promishification
0.0.6	Bugfixes and Documentation
4.2.2	Updated for compatability with Node v4. Version number matches the version of Node I used when this was published For compatability with older 0.x versions, specify Promish version 0.0.8 in your package.json Refactored to extend Promise instead of wrapping
4.2.3	Added implict Promish.all to spread
4.2.4	finally() must not swallow rejection.
4.2.5	jshint!
4.2.8	NPM Keywords