extended-promise

Promises are great! But, could they be even better?

Install

npm install @braintree/extended-promise

Why?

This library was developed to make working with APIs that are not easily wrapped in promises, such as kicking off a promise that resolves within a callback for an event listener.

Before we wrote this lib, we were saving references to the underlying promise, and references to the resolve/reject functions in order to resolve and reject it later:

// without this lib
class MyCustomObject {
  constructor() {
    this._promise = new Promise((resolve, reject) => {
      this._resolveFunction = resolve;
      this._rejectFunction = reject;
    });
  }

  asyncProcess() {
    // do something very async that's not easily wrapped in a promise
    if (success) {
      this._resolveFunction(data);
    } else {
      this._rejectFunction(new Error("fail"));
    }
  }

  methodRelyingOnResultOfPromise() {
    return this._promise;
  }
}

Instead, we can save a reference to just the promise, and then resolve or reject it directly:

// with this lib
class MyCustomObject {
  constructor() {
    this._promise = new ExtendedPromise();
  }

  asyncProcess() {
    // do something very async that's not easily wrapped in a promise
    if (success) {
      this._promise.resolve(data);
    } else {
      this._promise.reject(new Error("fail"));
    }
  }

  methodRelyingOnResultOfPromise() {
    return this._promise;
  }
}

Add a hook to run when a promise resolves/rejects

The object also supplies an onResolve and onReject hook that can be passed in on instantiation:

var promise = new ExtendedPromise({
  onResolve(data) {
    return someFunctionToTransformData(data);
  },
  onReject(error) {
    // decide if you want to catch the error
    return someFallbackDataInstead;
    // or if you want to continue through with the rejection
    return Promise.reject(error);
  },
});

Suppress unhandled promise rejection warnings

One of the benefits of using ExtendedPromise is that you can pass the promise object around and catch the errors asyncronously. However, this can result in Node or the browser assuming that when the promise rejects before you've added your rejection handler, that the promise is completely undhandled. To suppress these warnings, add the suppressUnhandledPromiseMessage option when instantiationg the promise:

var promise = new ExtendedPromise({
  suppressUnhandledPromiseMessage: true,
});

// do async stuff that results in the promise rejecting
// and don't worry about Node or the browser being
// angry about it

promise.catch(function (err) {
  // handle this later
});

You can also set this globally for all ExtendedPromises:

ExtendedPromise.suppressUnhandledPromiseMessage = true;

var promise = new ExtendedPromise();

// do async stuff that results in the promise rejecting
// and don't worry about Node or the browser being
// angry about it

promise.catch(function (err) {
  // handle this later
});

If both the global property and the instance property are set, it will use the instance property.

Use as a normal promise

When ExtendedPromise is instantiated with a function, it will return the underlying Promise. The resolve and reject instance methods should not be used with this method. This is simply to make the migration to using ExtendedPromise easier.

var promise = new ExtendedPromise(function (resolve, reject) {
  // if success
  resolve('value');

  // if failure
  reject(new Error('err'));
});

promise.then(val => {
  // handle result
}).catch(err => {
  // handle error
})

Use a custom promise implementation

If your environment does not support Promises, you can set the Promise object to be used directly on the Object.

const PromisePolyfill = require("promise-polyfill");

ExtendedPromise.setPromise(PromisePolyfill);

Development

Run tests:

npm test