Start Me Up: ts-loader meet .tsbuildinfo

With TypeScript 3.4, a new behaviour landed and a magical new file type appeared; .tsbuildinfo

TypeScript 3.4 introduces a new flag called --incremental which tells TypeScript to save information about the project graph from the last compilation. The next time TypeScript is invoked with --incremental, it will use that information to detect the least costly way to type-check and emit changes to your project.

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These .tsbuildinfo files can be safely deleted and don’t have any impact on our code at runtime - they’re purely used to make compilations faster.

This was all very exciting, but until the release of TypeScript 3.6 there were no APIs available to allow third party tools like ts-loader to hook into them. The wait is over! Because with TypeScript 3.6 the APIs landed: https://www.typescriptlang.org/docs/handbook/release-notes/typescript-3-6.html#apis-to-support---build-and---incremental

This was the handiwork of the very excellent @sheetalkamat of the TypeScript team - you can see her PR here: https://github.com/microsoft/TypeScript/pull/31432

What's more, Sheetal took the PR for a test drive using ts-loader, and her hard work has just shipped with v6.2.0:

• https://github.com/TypeStrong/ts-loader/pull/1012
• https://github.com/TypeStrong/ts-loader/pull/1017

If you're a ts-loader user, and you're using TypeScript 3.6+ then you can get the benefit of this now. That is, if you make use of the experimentalWatchApi: true option. With this set:

1. ts-loader will both emit and consume the .tsbuildinfo artefact.

2. This applies both when a project has tsconfig.json options composite or incremental set to true.

3. The net result of people using this should be faster cold starts in build time where a previous compilation has taken place.

ts-loader v7.0.0

We would love for you to take this new functionality for a spin. Partly because we think it will make your life better. And partly because we're planning to make using the watch API the default behaviour of ts-loader when we come to ship v7.0.0.

If you can take this for a spin before we make that change we'd be so grateful. Thanks so much to Sheetal for persevering away on this feature. It's amazing work and so very appreciated.