ThrottledQueue.js

function ThrottledQueue(){
    let queue = []
    let runningJobs = 0

    function nextJob() {
        runningJobs++
        setImmediate(() => {
            if (queue.length > 0)
                queue.pop().runJob() 
        })
    })

    function wrapJob(job) {
        let runJob
        let jobPromise = new Promise(resolve => {
            runJob = resolve
        }).then(job)

        let wrappedJob = {
            runJob: runJob,
            jobPromise: jobPromise,
        }

        jobPromise.finally(() => {
            runningJobs--
            nextJob(wrappedJob)
        })

        return jobPromise
    }

    return {
        push(job) {
            let wrapped = wrapJob(job)
            queue.push(wrapped)

            if (runningJobs <= 5) {
                nextJob()
            } 

            return wrapped.jobPromise
        }
    }
}