@villedemontreal/correlation-id/src/middleware/correlationIdMiddleware.ts

Express middleware to set a correlation in Express. The correlation id will be consistent across async calls within the handling of a request.

import * as express from 'express';
import { constants } from '../config/constants';
import { correlationIdService } from '../services/correlationIdService';

/**
 * Correlation ID Middleware
 *
 * @param filter a filter which is going to be called with the request to see if a
 * colleration id scope must be managed or not.
 */
export const createCorrelationIdMiddleware = (
  filter?: (req: express.Request) => boolean,
): ((req: express.Request, res: express.Response, next: express.NextFunction) => void) => {
  const correlationIdMiddleware = (
    req: express.Request,
    res: express.Response,
    next: express.NextFunction,
  ): void => {
    if (filter && !filter(req)) {
      next();
    } else {
      // ==========================================
      // If a Correlation ID (cid) exists in the request
      // it will be used. Otherwise, a new one is created.
      //
      // We add the cid to the response!
      // ==========================================
      let correlationId = req.get('X-Correlation-ID');
      if (!correlationId) {
        correlationId = correlationIdService.createNewId();
        (req as any)[constants.requestExtraVariables.cidNew] = correlationId;
      } else {
        (req as any)[constants.requestExtraVariables.cidReceivedInRequest] = correlationId;
      }
      if (!res.headersSent) {
        res.set('X-Correlation-ID', correlationId);
      }
      correlationIdService.withId(next, correlationId);
    }
  };

  return correlationIdMiddleware;
};