Allow minimal type checking of JavaScript files

I'm in the process of introducing TypeScript support into an enormous, 6-year old web application with a lot of JavaScript files. In order to get the most advantages out of TypeScript while we slowly migrate over (which will likely happen slowly), I'd like the following to be true:

• TypeScript files (.ts and .tsx) are type checked with strict: true, so that we get as much useful feedback about TS code as possible.
• JavaScript code that imports and then incorrectly uses typed TypeScript code throws a TS error
• Other TypeScript errors in JavaScript files are suppressed, because we haven't had the chance to migrate the file to TypeScript yet. OR, at the least, JavaScript files are type checked with strict: false to minimize the number of errors they output.

Primary Suggestion

It would be fantastic if typescript had a "minimalCheckJs" config option that, when set to true, would only check JS files for errors on imported typed code. For example, setting "minimalCheckJs": true, "strict": true in tsconfig.json would have this effect:

# moduleA.ts
function hello(msg) {} # throws "no implicit any" error on "msg" arg

export default function logA(msg: string) {
  console.log(msg);
}

# moduleB.js
import logA from './moduleA'
import something from './someJavaScriptFile.js' # does not throw "cannot find module" error

logA(1) # throws TS error, because logA is typed

function logB(msg) { # does not show "no implicit any" error on "msg" arg
  console.log(msg);
}

This feature would allow me to convert a file from JS to TS, add types to the file, and immediately be assured that the exported code is being used correctly throughout my entire code base (including JS files). Currently, I can set "checkJs": true, but then I will see thousands of other kinds of errors that TypeScript finds in the JS files, such as cannot find module errors.

Alternative Suggestion

If the above feature is difficult to implement, a less ideal but also helpful feature would be to allow setting "strict": false for JS files and "strict": true for TS files. Some way to combine the following:

# strictly type check TS files
{
  "compilerOptions": {
    "checkJs": false,
    "allowJs": true,
    "strict": true
  },
  "include": ["**/*.ts", "**/*.tsx"],
}

# type check JS files with strict: false
{
  "compilerOptions": {
    "checkJs": true,
    "allowJs": true,
    "strict": false
  },
  "include": ["**/*.js", "**/*.jsx"],
}

Start by examining how tsconfig.json options such as checkJs, allowJs, and strict are interpreted for JavaScript and TypeScript files. No source files, tests, or compiler entry points are identified in the issue, so trace the existing checkJs and strict handling first. Done means JavaScript reports errors from imported typed TypeScript code while suppressing unrelated JavaScript diagnostics, with coverage for the requested configuration.