@jasnell my personal interest in this API surface is a follow on from:

The goal being to facilitate test-coverage and other transpilation steps in .mjs files.

I'm picturing that one could instrument code for coverage using pseudo code that looks something like this:

export async function resolve(specifier, parentModuleURL, defaultResolver) {
  const resolved = new url.URL(specifier, parentModuleURL)
  const ext = path.extname(resolved.pathname)
  if (ext === 'mjs') {
    const source = fs.readFileSync(resolved.pathname)
    const instrumented = istanbul.instrument(source)
    const blob = new Blob([instrumentedSource], {type : 'application/mjs'})
    return {
      url: createObjectURL(blob),
      format: 'esm'
    }
  } else {
    return defaultResolver(specifier, parentModuleURL)
  }
}

Does it seem like I'm on the same page as to how this API could potentially be used?

...an aside:

I keep coming back to the argument that @guybedford's work on #15445 should be exposed through an API hook rather than just a flag. In the world of developer tools, it's often the case that a few transformations need to be performed in sequence, e.g.,

a TypeScript transpilation step takes place to translate TypeScript typing into valid ES2015
a Babel transpilation parsing bleeding edge features, e.g., class decorators.
Istanbul runs, adding line counters to each line of (now ES2015) code.

I don't hate the idea of using createObjectURL() to facilitate the transpilation step ... but now that I sit down and hammer out some pseudo code, I'm not immediately seeing how one could compose the multi-step transformations (described above) using the --loader flag.

In the land of require.extensions one is able to create a stack of the prior transformations being applied, and a multistep transpilation can be applied without each actor knowing about the other (this is important, given the fractal nature of developer toolchains).

CC: @demurgos, @iarna

Uh oh!

Implement createObjectURL/Blob from File API #16167

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