#Fn - Debugging

How can we debug errors in our Fn functions?

Let's imagine that we have deployed the following Nodejs code as a Fn function using Serverless:

const fdk = require('@fnproject/fdk');

fdk.handle((input) => {
  input = JSON.parse(input);
  let name = 'World';
  if (input.name) {
    name = input.name;
  }
  const response = { message: `Hello ${name}` };
  console.error(`I show up in the logs name was: ${name}`);
  return response;
});

And its corresponding Serverless YAML file:

service:
  name: hello-world

provider:
  name: fn

plugins:
  - serverless-fn
functions:
  hello:
    name: hello
    version: 0.0.1
    runtime: node
    format: json
    events:
      - http:
          path: /hello

Let's invoke correctly that function

serverless invoke --function hello --data '{"name":"Bob"}' -l

# Output
Serverless: Calling Function: hello
{ message: 'Hello Bob' }
I show up in the logs name was: Bob

If we were to call the above function with an incorrect json data you would get Hello World back instead of Hello Bob In order to debug this since there is nothing fatal happening and no stack trace is appearing in the logs you would need to add more console.error calls until you figure it out. If there were a major error that caused a stacktrace and the entire function to fail then you could easily call

serverless logs --function hello

That would print any stack traces that may have gone to stderr

This is a very basic example of debugging a Fn function, but it should hopefully highlight the basic principles. Obviously, in production environments, you would want to have more formal and sophisticated error handling built into your code.