I am getting below error.

internal/streams/legacy.js:59 throw er; // Unhandled stream error in pipe. ^ Error: Can't set headers after they are sent. at validateHeader (_http_outgoing.js:491:11) at ServerResponse.setHeader (_http_outgoing.js:498:3) at ServerResponse.header (D:\Demos\leaflet-map\node_modules\express\lib\response.js:767:10) at ServerResponse.send (D:\Demos\leaflet-map\node_modules\express\lib\response.js:170:12) at ServerResponse.json (D:\Demos\leaflet-map\node_modules\express\lib\response.js:267:15) at D:\Demos\leaflet-map\routes\users.js:18:9 at Stream.write (D:\Demos\leaflet-map\node_modules\event-stream\index.js:204:20) at Stream.stream.write (D:\Demos\leaflet-map\node_modules\through\index.js:26:11) at Stream.ondata (internal/streams/legacy.js:16:26) at emitOne (events.js:116:13) npm ERR! code ELIFECYCLE npm ERR! errno 1 npm ERR! leaflet-map@0.0.0 start: node ./bin/www npm ERR! Exit status 1 npm ERR! npm ERR! Failed at the leaflet-map@0.0.0 start script. npm ERR! This is probably not a problem with npm. There is likely additional logging output above. npm ERR! A complete log of this run can be found in: npm ERR! C:\Users\hmahajan\AppData\Roaming\npm-cache_logs\2018-07-23T11_54_11_153Z-debug.log

============================ users.js

var express = require('express');
var router = express.Router();
var request = require('request')
var JSONStream = require('JSONStream')
var es = require('event-stream')

router.get('/', function(req, res, next) {
  request({url: 'https://jsonplaceholder.typicode.com/posts/1'})
  .pipe(JSONStream.parse('*'))
  .pipe(es.mapSync(function (data) {
    res.json({
      "data":data,
      "sucess":"hello"
    });
  }))

});
module.exports = router;

==============================

what is the issue here??

Can't set headers after they are sent. at validateHeader (_http_outgoing.js:491:11)

This error is not coming from JSONStream. It's happening because in the es.mapSync callback you're writing data to the res by using res.json({ ... }). This happens multiple times if there are multiple items in the results from the request call.

Check the express documentation, but I'm pretty sure that res.json will send headers back to the client whenever it's called. Instead, you can try piping the data back:

router.get('/', function(req, res, next) {
  request({url: 'https://jsonplaceholder.typicode.com/posts/1'})
  .pipe(JSONStream.parse('*'))
  .pipe(es.mapSync(function (data) {
    return { 'data': data, 'success': 'hello' };
  }))
  .pipe(JSONStream.stringify())
  .pipe(res);

dominictarr / JSONStream

throw er; // Unhandled stream error in pipe. #157

============================ users.js