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tldr; I want to convert my JS project to TS one file at a time while being able to run Mocha tests without a build step.

I utilize a lot of Babel transforms (class props, jsx, ...) in my current javascript code, which Mocha handles at run-time by registering the babel loader (basically mocha --require @babel/register). This means running a single test is fast and requires no build step for the entire project.

I followed a guide on getting started with TypeScript using the (relatively) new babel plugin from Microsoft: @babel/preset-typescript. This worked fine for the basic case: converting app.js to app.ts.

What it didn't cover was how to do a step-wise transition. For me, fixing 3978 typescript errors (actual count after doing the find ...) is a bit overwhelming and would stall development for two weeks. Just getting my 200 LOC helpers lib to compile nicely with the definitions from react-redux took well over an hour.

While doing git mv app.{j,t}s worked fine, doing it to any other file was a disaster. Existing Mocha tests quickly crashed on being unable to find the right files, even when registering Babel and adding suitable extensions:

mocha --extension js,jsx,ts,tsx --require @babel/register

Typically if doing git mv Logger.{j,t}s I'd get Error: Cannot find module './lib/logging/Logger'.

Is there a way of getting Mocha's module loader to recognize typescript files and transparently run them through Babel?