Bump typescript from 3.2.4 to 3.8.2

[eps1lon]

3.2.4 is the lowest version of typescript we support. By holding it back we made sure all types work as expected with the lowest version.

We change this in order to verify efforts made to address #19113. The build will break at first. The changes to types will be included in a separate PR to verify that the changes work for 3.2.4 as well.

The goal is to have one PR that changes (and hopefully simplifies) the types without changing the typescript version. This PR will (after the previous one is merged) only contain changes to the tests and the used typescript version.

Changing this is justifiable because of precedent: we test with the latest version of react as well. So far this worked but it's hardly compareable since typescript and react use different version schemes.

[mui-pr-bot]

No bundle size changes comparing 7582461...3406717

Generated by 🚫 dangerJS against 3406717

[oliviertassinari]

@eps1lon There is an interesting data point we could leverage (but to be cautious about the interpretation, does it have a signal/noise ratio high enough?). @material-ui/pickers runs with a different version of TypeScript: v3.7.2. @dmtrKovalenko What have been your TypeScript versioning strategy so far? Did you notice pros/cons?

[dmtrKovalenko]

I was trying to be consistent with material-ui but then just updated to the latest one. And did not get any problems. Actually ts is not following semver. So I think we need to just update it, cause new features are really awesome.

[merceyz]

This would allow us to remove createStyles from all the demos

[eps1lon]

This would allow us to remove createStyles from all the demos

Sure but I don't think we should do it. Just like we wouldn't use any react features that aren't available in the lowest react version we support.

[oliviertassinari]

Should we add a mention about the lowest version of TypeScript we support in https://material-ui.com/getting-started/supported-platforms/ (after React)?

[merceyz]

Sure but I don't think we should do it. Just like we wouldn't use any react features that aren't available in the lowest react version we support.

Agreed, should probably mention somewhere that createStyles isn't needed for typescript >= 3.4
Perhaps in the JSDoc for the function itself

[eps1lon]

Intended to be long-living. Will receive occasional merges from other PRs that change types to make sure we're not creating debt.

[eps1lon]

Plan is to merge this once 3.8.2 is released.

[NMinhNguyen]

Curious how you plan on verifying that you don't accidentally use syntax not supported by 3.2.4? Is there some tool/setting that enables this?

[eps1lon]

Curious how you plan on verifying that you don't accidentally use syntax not supported by 3.2.4? Is there some tool/setting that enables this?

Not yet. If we regress too often we would have to look into it. Until then typescript is treated like any other dependency. For example we're also using the latest react version and have no tooling to verify how the code behaves on older versions.

[dmtrKovalenko]

BTW there is no critical breaking changes between 3.2 and 3.8 moreover if typescript will find something inconvenient it will fail to any if skipLibCheck: true.
So + to @eps1lon we should not verify users are updated to the same version as we are using.

[eps1lon]

there is no critical breaking changes between 3.2 and 3.8

But that is not for you to decide what's critical and what is not. If you use a particular pattern and this breaks between 3.2 and 3.3 then a red build might be critical because it prevents a deploy.

So we definitely keep supporting 3.2.4 in v4. Just like we support IE11 but don't actually run automated tests on it.

Only data point I have about usage is

Telemetry indicates 99.9% of VS Code users are 3.0 or newer

-- DefinitelyTyped/DefinitelyTyped#40050

I'd expect 3.2.4 or newer to cover at least 95%.