In JS, type annotations should not block errors from the rest of the program

In a Javascript file that is checked by flow, we may see code like this (from create-react-app/packages/react-error-overlay/src/utils/parseCompileError.js) ```js export type ErrorLocation = {| fileName: string, lineNumber: number, colNumber?: number, |} function parseCompileError(message: string): ?ErrorLocation { const lines: Array<string> = message.split('\n'); for (let i = 0; i < lines.length; i++) { // ................... return fileName && lineNumber ? { fileName, lineNumber, colNumber } : null; } ``` This behaves badly when the typescript compiler compiles it with checkJs on: **Expected behavior:** 1. Errors on the type declaration and all the type annotations. 2. However, ErrorLocation should be declared as a type alias, and message, lines, etc should all have their declared types. 3. If they are used incorrectly, they should have errors. **Actual behavior:** 1. Errors on the type declaration and all the type annotations. In the language service: 2. message, lines, etc have the correct types, **but ErrorLocation is type any**. 3. If they are used incorrectly, they have errors. In batch compilation: 2. No errors show up except those from (1), even if there are lots of other javascript files without type annotations.