Node.js: http.js:691 throw new Error('Can\'t set headers after they are sent.')

Question

Hello I am making simple application using MEAN stack. And I get this error http.js:691 throw new Error('Can\'t set headers after they are sent.') Here is the code: server.js ``` // set up var express = require('express'); var app = express(); var mongoose = require('mongoose'); // mongodb connection and app configuration mongoose.connect('mongodb://localhost/test'); app.configure(function () { app.use(express.static(__dirname + '/app')); app.use(express.logger('dev')); app.use(express.bodyParser()); app.use(express.methodOverride()); }); // define model var Employee = mongoose.model('Employee', { firstName : String, lastName : String, photoUrl : String }); // routes ========================== // api // GET app.get('/api/employees', function(req, res) { console.log("GET"); Employee.find(function(err, employees) { if(err) { res.send(err); } res.json(employees); }); }); // Get by ID app.get('/api/employees/:employeeId', function(req, res) { Employee.findById(req.params.employeeId, funct…

Accepted Answer

In

```
app.get('/api/employees/:employeeId', function() {...});
```

you are using res.json twice. Lets assume Employee.findById returns the results first. Then,

```
res.json(employee)
```

is already sent. Then the code enters Employee.find callback with a slight delay. This again triggers res.json(). But since the response to your request is already sent, it can't send another response to the same request.

This condition is throwing the error.

There are two ways to solve this.

- Nested callbacks - Call Employee.find inside Employee.findById callback.

- Add conditions in both the callbacks to check if other callback is complete. Then send a single response.

Node.js: http.js:691 throw new Error('Can\'t set headers after they are sent.')

Solution

Problem