isolatedModules - potential issue with @types?

[johnnyreilly]

Hello!

I'm one of the maintainers of ts-loader, a TypeScript loader for webpack. We've recently been doing some work to improve performance by switching ts-loader to transpileOnly mode and using the fork-ts-checker-webpack-plugin to perform type checking in a separate process.

As a further optimisation we've been thinking about making use of the isolatedModules flag to make ts-loader do even less work. However when testing this out @odensc has stumbled upon this error:

ERROR at /home/oden/Dev/JS/fttv/node_modules/@types/babel-core/index.d.ts(16,19):
TS1205: Cannot re-export a type when the '--isolatedModules' flag is provided.

I wanted to raise this to check if there some kind of issue using isolatedModules in conjunction with @types?

Also, I wanted to request guidance on use of isolatedModules - there isn't much documentation on the flag that I've been able to find. Will setting the flag to true affect the experience of users in vscode / visual studio / alm-tools etc? Will less errors etc be reported if this flag is set?

I ask as, historically, ts-loader has used the tsconfig.json as is and has done no "magic" with it. However, if by setting isolatedModules to true then we degrade the IDE experience then I can see a reason for deviating from this. i.e. Having ts-loader setting isolatedModules to true internally if a certain ts-loader config option is set. But is this necessary? Does this flag have any bearing on the IDE experience? I simply don't know at present I'm afraid.

cc @jbrantly @piotr-oles @odensc @basarat

Possible related issue: #2812

[basarat]

Also, I wanted to request guidance on use of isolatedModules - there isn't much documentation on the flag that I've been able to find. Will setting the flag to true affect the experience of users in vscode / visual studio / alm-tools etc? Will less errors etc be reported if this flag is set?

isolatedModules helps prevent writing code that might break if a file is compiled individually vs. part of a compilation context.

Essentially errors pointed out by isolatedModule would block a reliable emit on a file by file basis i.e. using ts.transpile(individualFile).

So by isolatedModule you get more errors. e.g. global namespace usages will become errors.

Example:
file a

namespace t{
   export const foo = 123;
}

file b

namespace t{
   log(foo);  // ERROR in isolatedModules
}

Because we need to emit log(t.foo) which is something we can only know if file a is a part of the compilation context.

PS: Always a please to hear from you @johnnyreilly ❤️ 🌹

TS1205: Cannot re-export a type when the '--isolatedModules' flag is provided.

My dream (might not be valid) would be that isolatedModules has no effect on .d.ts files as they are not re-emitted. But of course I can be wrong ❤️

[mhegazy]

@Andy-MS we should not check this if we are in ambient context, regardless of the compiler options.

[johnnyreilly]

Thanks for advising @basarat - as helpful as ever! 💓

So it seems like I have isolatedModules backwards then! It's actually not going to improve performance in ts-loader at all!

Following your explanation I'm now curious; do you know of a good use case for isolatedModules? Short of writing a repl I can't think of one (and I'm totally sure it earns its keep there). But doubtless this is solving a problem?

[basarat]

t's actually not going to improve performance in ts-loader at all!

True.

Following your explanation I'm now curious; do you know of a good use case for isolatedModules?

If you know you are going to compile your code using ts.transpile you can set isolatedModules to true to warn you about any TS that might generate potentially invalid JS 🌹

[johnnyreilly]

Cool - thanks 🌷

BTW ts-loader uses ts.transpileModule - is that any different to ts.transpile?

[basarat]

ts-loader uses ts.transpileModule - is that any different to ts.transpile?

Its the same. transpile is just a convenient wrapper around transpileModule and simply returns the outputText https://github.com/Microsoft/TypeScript/blob/441d76206578db4069c030b173ae660137532764/src/services/transpile.ts#L118 for when you don't care about diagnostics and sourcemaps 🌹

[johnnyreilly]

Sweet - you are a fount of knowledge!

[ghost]

If code is written to comply with --isolatedModules, it should be safe to transpile it without type information. (For example, you could use babel.) So this could have effects on performance. You could even transpile eagerly and type check using a separate process.