As a Node.js developer, you may have encountered the frustrating “TypeError app.listen is not a function” error when you are running a Node.js project.
This error usually occurs if you are trying to call the app.listen() method on an object which is not function.
In this article, we will explain to you in detail the reasons behind the “TypeError app.listen is not a function” error and how to fix it.
What Causes the “TypeError: app.listen is not a Function” Error?
The following are the most common reasons for this error:
- Incorrect Installation
- Incorrect Declaration
- Incorrect Version
- Invalid Object
How to Fix the “TypeError app.listen is not a Function” Error?
Now that we’ve discussed the possible reasons behind the “app.listen is not a function” error.
Next, we will discuss the solutions to fix it.
Solution 1: Check Your Installation
The first way to this error is to make sure that you have installed Express.js correctly.
Here’s how to check your installation:
- Open your project’s terminal or command prompt.
- Navigate to your project’s root directory.
- Run the following command: npm list express
An example after you run the npm list express:
Solution 2: Check Your Variable Declaration
If you are sure that the Express.js is installed propetly, you can check your variable declaration.
Make sure that you’ve declared the app variable properly through assigning it the result of calling the express() method.
const express = require('express'); const app = express();
The first line imports the Express library using the “require” function and assigns it to the variable “express”.
The second line creates a new instance of the Express framework by invoking the “express” function, and assigns it to the variable “app”.
Solution 3: Update Node.js and Express.js
When you are using an out-of-date version of Node.js or Express.js.
You will need to update to the latest version to resolve the error.
To update Node.js, you can visit the official Node.js website and download the latest version.
To update Express.js, just open your terminal or command prompt and run the following command:
npm update express
Solution 4: Check the Object Passed to app.listen() Method
If there is none of the above solutions working, check the object that you pass to the app.listen() method.
Make sure that you will pass a valid object that has a listen() method.
const http = require('http'); const express = require('express'); const app = express(); const server = http.createServer(app); server.listen(3000);
- The first line imports the built-in “http” module from Node.js and assigns it to the variable “http”.
- The second line imports the Express framework and assigns it to the variable “express”.
- The third line creates an instance of the Express framework and assigns it to the variable “app”.
- The fourth line creates an HTTP server using the “http.createServer()” method, passing in the “app” instance as an argument.
- The fifth line tells the server to start listening for incoming requests on port 3000.
- Typeerror: res.status is not a function
- Typeerror: path must be absolute or specify root to res.sendfile
In conclusion, the “TypeError app.listen is not a function” error is a common issue that Node.js developers may encounter. However, it can be resolved easily by following the solutions outlined in this article.
The app.listen() method is a built-in method in the Express.js library. It allows you to start a server and listen for incoming requests on a specified port.
Yes, you can specify a different port number when you call the app.listen() method. By default, the server listens on port 3000