RequestThrottler.ts

import moment = require('moment');

export class RequestThrottler {
  requestTimes: moment.Moment[];
  requestRateLimit: number;
  requestRateWindow: moment.Duration;
  pendingRequests: {
    resolve: (value?: any) => void;
    reject: (reason?: any) => void;
    request: () => Promise<any>;
  }[];

  timer: NodeJS.Timer;

  constructor(requestRateLimit: number, requestRateWindowInMilliseconds: number) {
    this.requestRateLimit = requestRateLimit;
    this.requestRateWindow = moment.duration(requestRateWindowInMilliseconds);
    this.requestTimes = [];
    this.pendingRequests = [];
  }

  async execute(request: () => Promise<any>): Promise<any> {
    if (this.currentRequestRate < this.requestRateLimit) {
      this.requestTimes.push(moment());
      return await request();
    } else {
      this.scheduleCheckPendingRequests();

      return new Promise<any>(async (resolve, reject) => {
        this.pendingRequests.push({
          resolve: resolve,
          reject: reject,
          request: request
        });
      });
    }
  }

  private async scheduleCheckPendingRequests() {
    this.timer = setTimeout(async () => {
      this.checkPendingRequests();
    }, 100);
  }

  private async checkPendingRequests() {
    if (this.pendingRequests.length === 0) {
      this.scheduleCheckPendingRequests();
      return;
    }

    if (this.currentRequestRate < this.requestRateLimit) {
      const pendingRequestsToExecute = this.requestRateLimit - this.currentRequestRate;

      let requestsToExecute = this.pendingRequests.slice(0, pendingRequestsToExecute);
      this.pendingRequests = this.pendingRequests.slice(pendingRequestsToExecute + 1, this.pendingRequests.length);

      for (let request of requestsToExecute) {
        try {
          this.requestTimes.push(moment());
          request.resolve(await request.request());
        } catch (error) {
          request.reject(error);
        }
      }

      this.scheduleCheckPendingRequests();
    } else {
      this.scheduleCheckPendingRequests();
    }
  }

  get currentRequestRate(): number {
    return this.requestTimes.filter(t => t.isSameOrAfter(moment().subtract(this.requestRateWindow))).length;
  }
}