Debug Vercel Serverless Functions Locally

This note is about how I managed to debug Vercel Serverless Functions locally.


If you prefer to watch the video instead of reading article,
its available at
Length: 05:22 mins


As the vercel functions are exporting a function which takes (req, res),
I imported this function in another file and called it by passing fake/empty (req, res).
For this, created a file src/debug.js with following content:

const api = require("../api/hello");
console.log("Debug started");
api({}, {});
console.log("Debug ended");

Debug config

Then added a file .vscode/launch.json, with following content:

  "version": "0.2.0",
  "configurations": [
      "type": "pwa-node",
      "request": "launch",
      "name": "Launch Program",
      "skipFiles": ["<node_internals>/**"],
      "program": "${workspaceFolder}\\src\\debug.js"

With the above file, I have now got a debug configuration in VS Code's Run and Debug (CTRL+SHIFT+D) side window.
Now I can click(play button) and start the debugging.

Do not forget to add the breakpoint(s) in your Vercel Function script file.

Fake more

At this point, I was able to debug and fix my code, but the API function didn't run till end as I was using res.json() method to return the response, which clearly was missing in our fake res object passed initially.

Debugging output:

Debug output

So to make it work, I passed fake json() method as part of res object, which just prints to the console.

Now the debug.js file looks like this

const api = require("../api/hello");
console.log("Debug started");
api({}, { json: (response) => console.log("RESPONSE", response) });
console.log("Debug ended");

Debugging now prints as below:

Debug output

That's it about this note.

~ @raevilman 🐦


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