TypeScript processor reads and parses tsconfig for each processed file; target is not respected

[cbxp]

Describe the bug
For performance reasons, tsconfig should be read only once per project. This can be verified in the debugger.
Also, target from tsconfig should be respected. Currently it is hardcoded to 'es6' no matter what tsconfig specifies.

Information about your project:

• svelte-preprocess version 4.7.3
• Whether your project uses Webpack or Rollup: Snowpack

Workaround for setting target in config:

preprocess({typescript: {compilerOptions: {target: 'esnext'}}})

However, something still re-processes it again, at least when using Snowpack. Will investigate more.

[kaisermann]

Hey, @cbxp 👋 There's definitely room for improving the tsconfig parsing. PRs are welcome! Unfortunately, I don't have the necessary time to focus too much on svelte-preprocess right now.

Now, for the target issue, that's a bit easier to fix. However, we have to check what is the most modern Javascript version the Svelte compiler supports. es6 was used right after the release of svelte v3

[dummdidumm]

Caching of the config is implemented, the target fix is pending.

[dimfeld]

It looks like Svelte is passing ecmaVersion: 12 to acorn, which according to the acorn docs corresponds to ES2021. So ES2021 could be the best default target setting. Probably worth testing with whatever features are new in ES2021 to make sure though.

Edit: Logical assignment operators (||= and &&=) and underscores for numeric separators appear to be the main syntactic changes. I'll try to get a PR in for this later this week.

[dummdidumm]

Bumping the target which unconditionally overwrites the user-defined target would be a breaking change, so the fix should contain logic that assigns the target if isn't present in the user's tsconfig.