How to debug a Node.js Server

You can grab the final demo app from my github profile.

How to use the node.js debugger

Node includes a built-in debugger that you can use on the command line. You can do most of the debugging actions that you normally do in the chrome DevTools or the VS Code debugger directly from the command line. Let’s create a very basic node HTTP server to try it out.

How to set up a basic node.js server

I’m using the following tools to setup the HTTP server in node.js:

• NPM v8.19.2 (you don’t actually need it, you can just run the commands through node)
• Node v20.10.0
• VS Code

You can follow these steps to get the server running

• Initialize the node project with npm init (you can use the default options for everything)
• Create an index.mjs file and paste in the following

    import http from "node:http";
    const PORT = 3000;
    const HOST_NAME = "localhost";
    
    async function fetchData() {
      return new Promise((resolve, reject) => {
        setTimeout(() => {
          resolve("Hello World");
        }, 3000);
      });
    }
    
    const server = http.createServer(async (req, res) => {
      if (req.url === "/") {
        const someDataFromDatabase = await fetchData();
        res.end(someDataFromDatabase);
      }
    });
    
    server.listen(PORT, HOST_NAME, () => {
      console.log(`Server running at http://${HOST_NAME}:${PORT}/`);
    });
    

• Here’s a brief overview of the code
  • we import the http module from node
  • we create a fetchData function which simulates getting data from somewhere else
  • we create the server with a handler that’s setup to handle only the root endpoint
  • we setup the server to listen to port 3000 on localhost
• You can now add this to the scripts section of the package.json file to setup a start command

    "start": "node index.mjs"

• Then, run the command through npm

  npm start

• when you visit localhost:3000 you should see hello world after 3 seconds.

How to start the node.js debugger

To start the node debugger, you have to add the inspect argument to your node command. So, you can add a start:debug command to the package.json script section like this:

"start:debug": "node inspect index.mjs"

If you run npm run start:debug then the debugger will pause on the first line of the file. To pause it on the debugger statement, you need to set the NODE_INSPECT_RESUME_ON_START variable to 1.

"start:debug": "NODE_INSPECT_RESUME_ON_START=1 node inspect index.mjs"

Now, let’s update out code to add a few debugger statements.

    import http from "node:http";
    const PORT = 3000;
    const HOST_NAME = "localhost";
    
    async function fetchData() {
      return new Promise((resolve, reject) => {
        setTimeout(() => {
          resolve("Hello World");
        }, 3000);
      });
    }
    
    const server = http.createServer(async (req, res) => {
      if (req.url === "/") {
        debugger;
        const someDataFromDatabase = await fetchData();
        debugger;
        res.end(someDataFromDatabase);
      }
    });
    
    server.listen(PORT, HOST_NAME, () => {
      debugger;
      console.log(`Server running at http://${HOST_NAME}:${PORT}/`);
    });

When you run the start:debug command again, you’ll see it pause after the server starts listening on the port.

    ❯ npm run start:debug
    
    > debugging-node-server@1.0.0 start:debug
    > NODE_INSPECT_RESUME_ON_START=1 node inspect index.mjs
    
    < Debugger listening on ws://127.0.0.1:9229/69001291-bc1e-4f0a-af8a-685cb2e7c74b
    < For help, see: https://nodejs.org/en/docs/inspector
    < 
    connecting to 127.0.0.1:9229 ... ok
    < Debugger attached.
    < 
    break in index.mjs:23
     21 
     22 server.listen(PORT, HOST_NAME, () => {
    >23   debugger;
     24   console.log(`Server running at http://${HOST_NAME}:${PORT}/`);
     25 });
    debug>

You can use the help command to see all the commands that you can use with this CLI debugger. The most useful ones will be c to continue and repl to go into the repl mode. Inside the repl mode, you can check the values of the variables in your program. Try to get to the last debugger statement and go into the repl mode to check the value of someDataFromDatabase. You just have to type the variable name and press enter in the repl mode to see it’s value. To actually reach that debugger line, you’ll also need to trigger the handler by visiting localhost:3000 in your browser.

You can checkout the full debugger docs on the node documentation to see what else you can do with it.

How to debug node.js using the chrome DevTools

If you’re already used to the chrome DevTools debugger, then you can also debug your node apps using that.

To setup debugging in chrome, you need to use the —inspect flag. Let’s setup another NPM command in your package.json to do this.

    "start:chrome-debug": "node --inspect index.mjs"

Now, when you run it in your terminal, you should see a similar message.

    ❯ npm run start:chrome-debug
    
    > debugging-node-server@1.0.0 start:chrome-debug
    > node --inspect index.mjs
    
    Debugger listening on ws://127.0.0.1:9229/8689da89-80f3-4cd4-bdd9-9a6199c8fc34
    For help, see: https://nodejs.org/en/docs/inspector
    Server running at http://localhost:3000/
    Debugger attached.

You can visit chrome://inspect in chrome to view your app using the chrome DevTools. You should see your app listed in the Remote Target section. Just click on inspect option and then you should see the chrome DevTools open up. Now you can debug your app with the chrome debugger.

How to debug node.js in VS Code

The VS Code debugger uses the CLI debugger under the hood and gives you the same options inside your editor. To start, you can go to the debugger window (Ctrl + Shift + D). If you’re doing this for the first time in your app then it will prompt you to create a launch.json file.

Click on that button to create a new launch.json file. For this specific demo, the default options should be fine. Go to your index.mjs file again and press the “Launch Program” button. It should pause at the first debugger line again and open up the debug console in your VS code terminal.

You can read more about the different options with the debugger on the VS code debugger docs.

I would recommend the following simple changes to the launch.json file for this demo server.

• Update the program property to your server entry file so the debugger will run that file regardless of what file you have currently opened . The default option will try to run the currently opened file with node instead.

    "program": "${workspaceRoot}/index.mjs",

• If you’re using something like NVM to manage your node versions then you can use the runtimeVersion property to match it to your version of node.

    "runtimeVersion": "20.10.0"