Switch to TSC transformer instead of Babel by default

[devongovett]

We should measure the performance impact of doing this first. However, based on issues, it seems fairly clear that the Babel-based TypeScript support that we have now is too limited. @babel/preset-typescript doesn't enable all of the language features that TSC does, e.g. decorators, class properties, optional chaining, etc. This causes confusion for people.

We could either attempt to match the features that TSC supports in our default config, or just run TSC first, and then Babel like we did in v1. The main issue is that when you add a custom Babel config, you'd need to re-create our default yourself, which might also be confusing.

The action items here are:

1. Switch a large TypeScript project to use @parcel/transformer-typescript-tsc for .ts and .tsx files instead of babel, and measure the performance impact.
2. If the perf impact is manageable, switch the default config over to TSC. Otherwise, discuss other options.

[mikeplus64]

I wonder if this is the best criteria for deciding if TSC is OK as the default. Mainly, I would be surprised if there are actually any large TypeScript projects that didn't have a tsconfig / use TSC already.

[devongovett]

Having a tsconfig and using TSC for compilation are two different things. For example, you could have a tsconfig for type checking and editor integration, but use Babel for compilation. This is how popular tools like create-react-app and Next.js work.

[aminya]

I prefer tsc since it is "the main typescript" compiler. Build time is not a concern for me. I have never had any compiling-time issues in JavaScript. When you consider the time it takes to build a native application using LLVM for example, tsc vs babel becomes just negligible. You should not sacrifice the quality of the code and development just to cut the build time!
tsc has "incremental building" and "watch mode" built-in, which can increase the build performance.

[LukeTOBrien]

tsc - Compiling TypeScript as nature intended.

[lgarron]

1. Switch a large TypeScript project to use @parcel/transformer-typescript-tsc for .ts and .tsx files instead of babel, and measure the performance impact.

Suppose that we tried this on a moderately sized project and it failed. Is it worth filing bugs now, or if/when tsc is the default?

[mischnic]

Is it worth filing bugs now

Yes

[lgarron]

Is it worth filing bugs now

Yes

I've given this a a go again, and this time I think I'm getting a TypeScript 3 compat error? Is that expected / worth filing an issue for?

npm ERR! code ERESOLVE
npm ERR! ERESOLVE unable to resolve dependency tree
npm ERR! 
npm ERR! While resolving: [email protected]
npm ERR! Found: [email protected]
npm ERR! node_modules/typescript
npm ERR!   dev typescript@"^4.0.2" from the root project
npm ERR! 
npm ERR! Could not resolve dependency:
npm ERR! peer typescript@"^3.0.0" from @parcel/[email protected]
npm ERR! node_modules/@parcel/transformer-typescript-tsc
npm ERR!   @parcel/transformer-typescript-tsc@"2.0.0-nightly.625" from the root project
npm ERR! 
npm ERR! Fix the upstream dependency conflict, or retry
npm ERR! this command with --force, or --legacy-peer-deps
npm ERR! to accept an incorrect (and potentially broken) dependency resolution.
npm ERR! 
npm ERR! See /Users/lgarron/.cache/npm/eresolve-report.txt for a full report.

npm ERR! A complete log of this run can be found in:
npm ERR!     /Users/lgarron/.cache/npm/_logs/2021-03-11T11_40_11_289Z-debug.log

[mischnic]

@lgarron I think that's already reported here: #5978?

[devongovett]

An update on this: We are now transpiling TypeScript with SWC by default as of #6230. It is an order of magnitude faster than both Babel and TSC. It also fixes several issues that people had with using Babel as a transpiler for TypeScript. I think this changes the equation for what should be the default in Parcel significantly. Perhaps we should attempt to read tsconfig.json and apply the settings to SWC, as well as enable TS-specific transforms by default (e.g. decorators), but I no longer think using TSC by default is a good idea as it would harm performance significantly.

[devongovett]

As of #6501 we are loading some settings from tsconfig.json and applying them via SWC. Currently we support settings for JSX and enabling decorators, which are the only compiler features that have to be enabled and are not part of the default configuration. As more experimental features are added, we will also support those.

Going to close this issue. If you want to use TSC, you still can by using @parcel/transformer-typescript-tsc.