Write fewer lines of code by turning node-style asynchronous functions and promises to a synchronous code using ES6 generators.

few is an npm module intended to run on node.js 4.0.0 and higher.
This package is continuously tested on all minor versions from node.js 4.0.0 and higher using Travis CI.

Using npm:

npm install few

Using bower:

bower install few

Supporting all modern browsers using Babel.

const few = require('few');

function returnValue(v, callback) {
  process.nextTick(() => callback(null, v));
}

function* generateValue(v) {
  return yield cb => returnValue(v, cb);
}

// Multiple invocations of few run asynchronously

few(function* () {
  // Yield or delegate directly
  const a = yield cb => returnValue(1, cb);
  const b = yield Promise.resolve(2);
  const c = yield 3;
  const d = yield* generateValue(4);  // yield generateValue(4); Also works

  // Prints 1 2 3 4
  console.log(a, b, c, d);
});

few(function* () {
  // Parallelize using arrays
  const arr = yield [
    cb => returnValue(1, cb),
    Promise.resolve(2),
    3,
    generateValue(4)
  ];

  // Prints [ 1, 2, 3, 4 ]
  console.log(arr);
});

few(function* () {
  // Parallelize using objects
  const obj = yield {
    a: cb => returnValue(1, cb),
    b: Promise.resolve(2),
    c: 3,
    d: generateValue(4)
  };

  // Prints { a: 1, b: 2, c: 3, d: 4 }
  console.log(obj);
});

genOrFn must be an initialized generator or a generator function that does not expect any arguments. Any other type will produce a TypeError.
callback, if provided, must be a node-style callback, i.e. accepting an error and a result as arguments.
The return value of the generator will be provided as the result argument and if an error is thrown, it will be provided as the error argument.
If callback is not provided, any error that the generator produces, will be thrown.

few supports the following types to be yielded:

• Single node-style callback argument functions (aka thunks)
• Promises
• Simple values, which will be returned as-is
• Arrays combining thunks, promises, generators or simple values to be run in parallel
• Objects containing thunks, promises, generators or simple values to be run in parallel

few allows parallelization by yielding an array or an object.
The yielded object or array may contain any combination of:

• Single node-style callback argument functions (aka thunks)
• Promises
• Initialized generators
• Simple values, which will be returned as-is

When all given elements have finished processing, a new object or array that contains the results of the given elements in the same order will be returned.
If any of the elements provides an error, the error will be thrown inside the generator.

Delegation is supported using the yield* expression.
To run in parallel, a generator can be passed as an element of the yielded array.

Any error originating from yielded objects will be thrown inside the generator, and can be caught using try...catch.
For example, the following code prints ERROR to stderr:

few(function* () {
  try {
    yield Promise.reject(new Error('ERROR'));
  } catch (err) {
    console.error(err.message);
  }
});

If an error is thrown inside a generator (and not caught), it will be passed to the callback given as a second argument to few or thrown if a callback has not been given.
The following example also prints ERROR to stderr:

few(function* () {
  yield Promise.reject(new Error('ERROR'));
}, (err, result) => { console.error(err.message); });

In the following example, the uncaught error will crash the process. Make sure you handle all errors!

few(function* () {
  yield Promise.reject(new Error('ERROR'));
});

Licensed under Apache 2.0