Config option for reporting full-stack code coverage

[PaulKiddle]

Body

Summary

Add a config option to astro dev and/or astro preview that:

• Instruments client-side code so that coverage can be collected by browser test tools like Cypress
• records server-side code coverage (using a library like C8 or Node's built-in code coverage flag)
• reports or outputs this server-side coverage to a file

Background & Motivation

Code coverage is a useful tool for developers to (1) ensure they have a broad automated test suite and (2) identify unreachable or otherwise unused code and branches.

Currently Astro users who want to record full-stack code coverage must:

• enable source map generation and Istanbul instrumentation for client-side code in the vite config object
• start the astro server using c8 or a similar library

These options are not well documented and buggy (e.g. c8 cannot record coverage when a process exits using process.exit(), so users must use middleware mode and start/stop the server programmatically; also the source maps don't quite align with the source .astro files).

(Edit: the vite istanbul plugin doesn't actually instrument inline script tags from .astro files, so further configuration is needed - I'm not actually sure how one would do this)

By providing first-party support for code coverage Astro can help developers make their code more robust and well-tested.

Goals

• Help developers configure code coverage
• Help developers understand how much of their code has been tested and where the gaps are

Example

Astro config option:

import { defineConfig } from 'astro/config';
import node from '@astrojs/node';

export default defineConfig({
  output: 'server',

  adapter: node(),

  collectCoverage: true
});

Using cli flags:

npx astro dev --collectCoverage