[Case Study] Can't find tslib with rollup-plugin-typescript2

[brandon-leapyear]

What package is covered by this investigations?

• rollup
• rollup-plugin-typescript2

Describe the goal of the investigation

Why does rollup fail?

Investigation report

Related: ezolenko/rollup-plugin-typescript2#12
Minimal repro: https://github.com/brandon-leapyear/yarnpkg-berry-repro-2140

I'm getting an error like

[!] (plugin rpt2) Error: /Users/bchinn/Desktop/foo/index.ts(3,16): semantic error TS2354: This syntax requires an imported helper but module 'tslib' cannot be found.

Which is odd to me because rollup-plugin-typescript2 declares tslib as a dependency (link). I can solve the error by adding tslib to my own devDependencies, but I'm wondering if that's the actual solution, or if there's something else going on.

[SagnikPradhan]

Probably because the plugin uses your work-space's typescript which does not have access to the plugin's dependencies?

[brandon-leapyear]

✨ This is an old work account. Please reference @brandonchinn178 for all future communication ✨

---

Even if the plugin is the one calling require('typescript')?
https://github.com/ezolenko/rollup-plugin-typescript2/blob/5ce7676ad0451df81e61190d42694f97085111fc/src/index.ts#L73

[SagnikPradhan]

Yep, because plugin has typescript declared as peer dependency.

[brandon-leapyear]

✨ This is an old work account. Please reference @brandonchinn178 for all future communication ✨

---

I see. So unless the plugin moves typescript into dependencies, there's no way around the user needing to install tslib? If that's the case, I'll make a PR on the plugin adding this step for pnp environments

[merceyz]

I can solve the error by adding tslib to my own devDependencies, but I'm wondering if that's the actual solution, or if there's something else going on.

That is the correct solution, TypeScript is trying to resolve tslib from your package so it is the one that needs to declare it. The same is true for other package managers but hoisting magically makes it work though it's inherently broken https://yarnpkg.com/advanced/rulebook#packages-should-only-ever-require-what-they-formally-list-in-their-dependencies