Promise Limiter, somewhat similar to https://github.com/d3/d3-queue or https://github.com/natevw/fifolock but with async Promises instead of callbacks

Politer.mjs

/*

import Politer from "./this_gist.mjs";

let q = Politer(2),
  politelyFetch = q.wrap(fetch);

Promise.all([
  politelyFetch(req1),
  politelyFetch(req2),
  politelyFetch(req3),
  politelyFetch(req4),
  politelyFetch(req5),
]).then(/* … */);

*/

export default class PromiseLimiter {
  constructor(n) {
    this._max = n;
    this._wip = 0;
    this._q = [];
  }
  
  _proceedIfReady() {
    if (this._q.length && this._wip < this._max) {
      ++this._wip;
      let task = this._q.pop();
      task.fn.apply(task.ctx, task.args)
        .then(result => { task.resolve(result); })
        .catch(error => { task.reject(error); })
        .finally(() => {
          --this._wip;
          this._proceedIfReady();
        });
    }
  }
  
  wrap(fn) {
    let scope = this;
    return function () {
      let task = {fn, ctx:this, args:arguments};
      let asyncResult = new Promise((resolve, reject) => {
        Object.assign(task, {resolve,reject});
      });
      scope._q.unshift(task);
      scope._proceedIfReady();
      return asyncResult;
    }
  }
}