Consider using Typescript strict flag

[danilofuchs]

Great job on the Typescript Migration!

strict is one of the most useful Typescript flags to catch cannot access key of undefined errors early on.

Flag documentation

https://www.typescriptlang.org/tsconfig#strict

Migration process

Having worked on migrating a big non-strict project to strict mode, I know it is quite the undertaking. Typescript does not allow to apply a flag to a single file, so enabling it directly would mean porting the entire codebase at once, which is very error-prone.

However, I have used ts-strictify to run an additional CI check if enabling strict flag would break any changed files. This way, we can be sure any new code is strict.

[jackfranklin]

Would definitely like to do this and it's unfortunate we can't turn it on file by file.

I wonder if rather than immediately leaping to strict: true, if we could turn on all the flags that strict: true implies one-by-one in PRs? That way we might not have to fix ~400 errors all at once and might be able to bring it down.

[OrKoN]

We could turn the flag on locally and fix issues, then submit those patches with strict: false. In the end, we'd hopefully only need to fix a few regressions in the final PR that turns on strict true. We can also go by individual components but I am afraid most of errors are about getting data from hash maps.