Transpile source files in the main process

[novemberborn]

To improve startup time (#78) we could provide a framework where source files are optimistically transpiled and cached in the main process. No require hooks would be required in the test files avoiding the Babel startup penalty. Thanks @tomazzaman for the suggestion.

• --require hooks won't do, we should support plugins and wrap them like how caching-precompiler wraps Babel
• The watcher needs to recompile changed source files before tests are rerun
• The test worker won't know which cache misses should have been precompiled, so tests may blow up. This would probably be due to insufficient --source patterns so would require useful error messaging somehow

[vadimdemedes]

Am I confused or we already do this via #390?

[novemberborn]

#390 transpiles tests files. This is suggesting infrastructure to transpile source files in the main process as well.

[vadimdemedes]

Yeah, but we don't transpile source files at all now. So we may start with that.

[novemberborn]

Yes, that's the point of this issue. We'd have to use the approach from caching-precompiler to get the compiled sources into the child processes, and we have to use glob patterns to match all potential sources before tests run so they're available synchronously.

[jamestalmage]

we have to use glob patterns to match all potential sources ...

Or use babel-detective which is probably more performant (the AST is being scanned already, so it should be faster than a regexp). I created it with this idea in mind.

Either way this won't do much good for dynamic requires. I did experiment with this, which could be used as a fallback for dynamic require calls.

I think a first step should be supporting transpilation of helper files (files in the test directory prefixed with _, or in test/helpers). Sources would get a different Babel transform without power-assert, so helpers seems like an easier place to start.

[jamestalmage]

For "non-Babel" transpilers (typescript, etc), we would have to create a plugin ecosystem. We would still need to finish up with Babel for powerAssert support.

Conceivably, users may want to chain custom transforms.

See transform-chain. It could help with all this.

[novemberborn]

This will be tackled through RFC 001. See the project here: https://github.com/avajs/ava/projects/1.

[sliekens]

@novemberborn link is dead so I'm not sure where to look

[novemberborn]

Yea, I pulled the RFC recently. #1908 is probably the best issue to track for now.