Cough: well, that broke all the CI! Some reading to do...

Bump the minimum node version for engine, and tests. Pinned back rollup-plugin-ts after getting errors on node 14.

Update, rollup-plugin-ts v3.2.0 dropped support for node 14 (oops!) : wessberg/rollup-plugin-ts#202

The official rollup typescript plugin appears to work fine (tests pass) and supports "node": ">=14.0.0", so will try switching to that.

https://www.npmjs.com/package/@rollup/plugin-typescript

Coverage dropped slightly and failed CI check, presumably because some extra code included by different typescript plugin. I have not been brave enough to try a diff on the output yet, but now might be the time...
Edit: generating identical cjs code, apart from sourceMap comment for "test" build.

Note to self: run more of the checks before pushing! I am used to editor catching the lint problems...

Some interaction between tsconfig sourceMap and rollup sourcemap and plugin configuration meant maps were being generated. Added an explicit passthrough of the local configured setting in rollup configuration.

Edit: removed override. Generating sourcemap is expected for test due to env?

    "pretest": "rimraf build && tsc -p tsconfig.test.json && cross-env NODE_ENV=test npm run build:cjs",

Pulled this branch down locally and it's working for me to compile and run tests.

Rollup supports setting environment variables through its CLI, meaning we can get rid of cross-env:

- "pretest": "rimraf build && tsc -p tsconfig.test.json && cross-env NODE_ENV=test npm run build:cjs",
+ "pretest": "rimraf build && tsc -p tsconfig.test.json && npm run build:cjs -- --environment NODE_ENV:test",

This could also be simplified to just a TEST environment variable:

// package.json
- npm run build:cjs -- --environment NODE_ENV:test
+ npm run build:cjs -- --environment TEST

// rollup.config.js
- if (process.env.NODE_ENV === 'test') output.sourcemap = true
+ if (process.env.TEST === 'true') output.sourcemap = true

Thanks @tommy-mitchell. I will look into those suggestions.

I like the --environment approach to drop a dependency and use the rollup support.

(The pattern of passing options into another run-script is a little unfamiliar to me, but I am coming around to the idea.)

Reminded by the failure that the coverage fails because it is being run against the test build which adds a last line to file:

//# sourceMappingURL=index.cjs.map