@antora/assembler/lib/util/promise-queue.js

A JavaScript library that merges AsciiDoc content from multiple pages in an Antora site into assembly files and delegates to an exporter to convert those files to another format, such as PDF.

'use strict'

class PromiseQueue {
  #scheduler
  #started
  #pending
  #rejection
  #startTask
  #trapRejection

  constructor ({ concurrency = Infinity } = {}) {
    this.concurrency = concurrency
    this.#scheduler = Promise.resolve()
    this.#started = []
    this.#pending = []
    this.#startTask = (task) => {
      if (this.#rejection) return
      if (this.concurrency === Infinity) {
        this.#started.push(task().catch(this.#trapRejection))
      } else {
        let current
        this.#pending.push(
          (current = task()
            .catch(this.#trapRejection)
            .finally(() => this.#pending.splice(this.#pending.indexOf(current), 1)))
        )
        this.#started.push(current)
      }
    }
    this.#trapRejection = (err) => (this.#rejection = err || new Error()) && undefined
  }

  add (tasks) {
    if (!Array.isArray(tasks)) tasks = [tasks]
    for (const task of tasks) {
      if (this.#pending.length < this.concurrency) {
        this.#startTask(task)
      } else {
        this.#scheduler = this.#scheduler.then(
          () => this.#pending.length && Promise.race(this.#pending).then(() => this.#startTask(task))
        )
      }
    }
    return this
  }

  async toPromise () {
    // what about promiseAll(), all(), or promise()?
    await this.#scheduler
    return Promise.all(this.#started).then((returnValues) => {
      if (this.#rejection) throw this.#rejection
      return returnValues
    })
  }
}

module.exports = PromiseQueue