pawc

pawc (Promise All with Concurrency)

This is basically a Promise.all() but with limiting the concurrency. It works where you pass it the concurrency, and a promise map and will either resolve with a map of resolve and reject maps that will match your promise map or optionally reject on the first rejected promise. A promise map is an object with keys that id the promise and the value is a function that returns a promise (!! Important the value is a function that returns a promise NOT a promise itself).

A neat side-effect of using this, is if you set the concurrency to 1 then it will sequentially run your promises.

Install

just run: $ npm i pawc

Usage

Simple Example

Super simple example, it's from a test in tests.js.

async function () {
  // map of two promises
  const promisesMap = {
    a: () => Promise.resolve('hello from a'),
    b: () => Promise.resolve('hello from b'),
  }
  const result = await pawc.all(1, promisesMap) // result is { resolves: { a:'hello from a', b:'hello from b' }}
}

Another simple example with a rejection, it's also from a test in tests.js.

async function () {
  // map of two promises
  const promisesMap = {
    a: () => Promise.resolve('hello from a'),
    b: () => Promise.reject(new Error('opps')),
  }
  const result = await pawc.all(1, promisesMap)  // result is { resolves: { a:'hello from a' }, rejects: { b: [oops Error] } }
}

Reject on First

If you just want to reject the whole lot of promises on the first rejection then pass true for the rejectOnFirst parameter (it is false by default). Since this is a reject and returns an error, you will lose any resolves that finished before the first rejection.
Here is another example from the tests.js:

async function () {
  const promisesMap = {
    a: () => Promise.reject(new Error('oops')),
    b: () => Promise.resolve('will not be called') }
  }
  try {
    await pawc.all(1, promisesMap, true)
    // never reaches here
  } catch (error) {
    console.error(error.message) // oops
  }
}

Async Await Example

Check out example.js for a little more complex example using async functions and await. You can run it with DEBUG=* node example.js to see extra debug logging. Here is the example output when it is ran:

pawc pawc.all called with concurrency of 2 and 5 promises ("task #1", "task #2", "task #3", "task #4", "task #5") +0ms
pawc starting promise task #1 +1ms
pawc starting promise task #2 +1ms
pawc promise task #2 finished +2s
pawc starting promise task #3 +0ms
pawc promise task #1 finished +2s
pawc starting promise task #4 +0ms
pawc promise task #4 finished +1s
pawc starting promise task #5 +0ms
pawc promise task #5 finished +1s
pawc promise task #3 finished +1s
pawc all promises done +0ms
{
  resolves: {
    'task #2': { id: 'task #2', ms: 1781.560542552069 },
    'task #1': { id: 'task #1', ms: 3641.017426104276 },
    'task #4': { id: 'task #4', ms: 1174.2142358962817 },
    'task #5': { id: 'task #5', ms: 1170.231974065761 },
    'task #3': { id: 'task #3', ms: 5306.500226395915 }
  },
  rejects: {}
}

Version History

• 1.0.0 - initial release
• 1.0.1 - example and doc fixes
• 1.1.0 - fix using multiple instances of pawc