Document Declarative Function Signatures

[grant]

We should document the interface to declarative function signatures on the main README.md. It's not clear how to use the interface or how it interacts with non-declarative functions. There are many edge cases to consider, examples:

• What happens if there are multiple functions with the same name?
• What are valid function names? Do these align with GCF?
• What if both a declarative and non-declarative function has been specified?

Implementation: #347

[grant]

From what I can tell, the current implementation doesn't work as expected.

const FunctionsFramework = require('@google-cloud/functions-framework');

FunctionsFramework.http('function', async (req, res) => {
  res.send('Hello, World!');
});

Command:

functions-framework-nodejs

Error:

Function 'function' is not defined in the provided module.
Did you specify the correct target function to execute?
Could not load the function, shutting down.

[matthewrobertson]

From what I can tell, the current implementation doesn't work as expected.

For your example, you would still need to provide the target cli argument:

functions-framework-nodejs  --target=function

The fallback to the default function target is only implemented for non-declarative functions. I didn't included it in the declarative API because there is already a TODO questioning it's existence. As it is today, I don't think it is possible to use the default on GCF because you must provide a target name when you deploy your function.

Also function is a keyword in javascript, so it is a really weird thing to name a method (I'm not even sure it is allowed).