Disable `--noImplcitAny`, `--noImplicitThis` and `--strictNullChecks` in .js files

[mhegazy]

• Implicit Any errors should be disabled in .js even if --noImplicitAny is set for the whole compilation.
• noImplicitThis inferences should be enabled, but errors disabled
• Similarly, inferences for --strictNullChecks for null and undefined types should stay, but errors like Variable is used before being assigned or Object is possibly undefined should be disabled.

This allows for users to mix and match .ts and .js files, and enable strict checks for the .ts files without having errors in the .js file

[Jessidhia]

Variable is used before being assigned is definitely useful in .js files when used with const. The let case is not as easy, but will the compiler still be able to do TDZ inference?

[mhegazy]

The compiler can not get TDZ right all the time. in .ts files if the compiler misses, users can use the non-null assertion operator !, but that is not possible in a .js file, hence disabling the error.

for instance:

var x;
setx();
x++;

function setx() { 
    x = 0;
}

So either we report the error, and users can not disable it, or not report the error at all.

[Jessidhia]

What I'm wondering is whether TypeScript will be able to flag this specific error:

x // ReferenceError
let x // or `class x`

var doesn't create a TDZ so it's fine to just consider it undefined.

[mhegazy]

that is a different error. block scoped variables are flagged if used before they are declared in general, this is not impacted by --strictNullChecks.