Run promises concurrently, wait until they are all settled

concurrently.js

const runConcurrently = concurrency => factories => {
    const queue = [];

    const lock = () => new Promise(resolve => {
        if (concurrency > 0) {
            concurrency--
            resolve()
        } else {
            queue.push(resolve)
        }
    })

    const run = factory => () => {
        const promise = factory()

        return promise.catch(() => Promise.resolve()).then(() => promise)
    }

    const unlock = result => {
        if (queue.length) {
            queue.shift()()
        } else {
            concurrency++
        }

        return result
    }

    return factories.map(factory => lock().then(run(factory)).catch(rejected => (unlock(), Promise.reject(rejected))).then(unlock))
}

const settleAll = promises => Promise.all(promises.map(promise => promise.catch(() => null))).then(() => promises)

// Test code follows

const workload = (label, success, result, delay) => () => new Promise((resolve, reject) => {
    console.log(`Workload started: ${label}`)

    setTimeout(() => {
        console.log(`Workload completed: ${label}`)

        return success ? resolve(result) : reject(result)
    }, delay)
})

let workloads = [workload('A', true, 1, 1000), workload('B', false, 2, 1000), workload('C', true, 3, 1000), workload('D', false, 4, 1000), workload('E', true, 5, 1000)];

settleAll(runConcurrently(2)(workloads)).then(() => console.log('all done!'))

// Profit