Enforce Function File Contract earlier

[WilliamBergamin]

Summary

The goal of this PR is to allow the deno slack builder to detect if a function file has a default export and notify the user is it does not.

ticket: HERMES-2867

Test:

1. Create a Deno project using the CLI or cd to an existing project
2. Run

   deno run -r --allow-read --allow-write --allow-net --allow-run --config=deno.jsonc 
   https://raw.githubusercontent.com/WilliamBergamin/deno-slack-builder/early-function-contract/src/mod.ts

   everything should pass
3. Edit a file containing a Function and remove the export default handler
4. Run

   deno run -r --allow-read --allow-write --allow-net --allow-run --config=deno.jsonc 
   https://raw.githubusercontent.com/WilliamBergamin/deno-slack-builder/early-function-contract/src/mod.ts

   This time something should fail and the output should contain this

   error: Uncaught (in promise) Error: File: {YOUR PATH TO BROKEN FILE}, containing your function does not define a default export handler.
   

I'm looking for feedback on additional testing, making sure this will work with the other components of the system

Requirements

• I've read and understood the Contributing Guidelines and have done my best effort to follow them.
• I've read and agree to the Code of Conduct.

[codecov]

Codecov Report

Merging #27 (99d8fe7) into main (daf2b41) will increase coverage by 20.52%.
The diff coverage is 100.00%.

@@             Coverage Diff             @@
##             main      #27       +/-   ##
===========================================
+ Coverage   10.60%   31.13%   +20.52%     
===========================================
  Files           4        4               
  Lines         198      212       +14     
  Branches        0       12       +12     
===========================================
+ Hits           21       66       +45     
+ Misses        177      145       -32     
- Partials        0        1        +1     

Impacted Files	Coverage Δ
src/dev_deps.ts	100.00% <100.00%> (ø)
src/functions.ts	39.82% <100.00%> (+38.82%)	⬆️

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[WilliamBergamin]

Need to do more testing before reopening this PR

[filmaj]

LGTM! Left a few small nitpicks, looks good otherwise. Any larger tasks coming out of any of my comments can be filed as separate issues and addressed later.

[filmaj]

Small suggestion, the flags should go before the directory parameter, just to align with deno test CLI help/usage docs.

Suggested change

	"test": "deno fmt --check && deno lint && deno test src --allow-read",
	"test": "deno fmt --check && deno lint && deno test --allow-read src",

[filmaj]

Random thought: we could potentially enforce the type of default export returned by userland function code, ensuring that they are e.g. exporting the return of SlackFunction.

Not a blocker for this PR but just food for thought / future enhancement.

[filmaj]

Thinking more about how these default exports are consumed at runtime, and in particular:

• At runtime, if no default export exists, the runtime will throw an exception, and
• The runtime directly invokes the default export as a function,

Perhaps we should ensure that the default export is, at least, a function? Maybe not as far as my previous comment suggested (to check the specific return type), but maybe as simple as a typeof functionModule.default == 'function' check?

Adding this additional check would prevent a developer from doing something silly like export default 3; or some other non-function type.

[shapirone]

Good call to ensure the default export is a function, like that the footprint is minimal

[WilliamBergamin]

Ive tested and implemented the typeof functionModule.default == 'function' check good idea @filmaj 💯

[filmaj]

I am a bit of a pedant when it comes to test names - I really like for the test name to make clear what the assumptions around a test are, as well as what the expected behaviour is. I'd change this to something like:

Suggested change

	Deno.test("Function files should have a default export", async () => {
	Deno.test("validateAndCreateFunctions should not throw an exception when fed a function file that has a default export", async () => {