3.1rc (js) regression: writing to property of a global variable redefines entire variable type

[brendankenny]

TypeScript Versions: 3.1.0-rc.20180911, 3.1.0-dev.20180914

Not an issue in 3.1.0-dev.20180831 (bisected to #26908)

Search Terms:
does not exist global write module

Code

// test.js
const INPUT = process.argv;
process.env.ENV_VAR = 'val';

module.exports = INPUT;

Run tsc --noEmit --allowJs --checkJs with @types/node available (see below for a non-node example).

Expected behavior:
No error.

Actual behavior:
error TS2339: Property 'argv' does not exist on type 'typeof process'.

Not a problem in typescript code. When all the example lines are present, the definition and type definition of process now point to the process.env.ENV_VAR line instead of to @types/node.

To get the bug, either module.exports needs to be set or a file needs to be required or imported, so I assume it being a module is important. Writing to anything on process seems to trigger it, not just a sub-property (so e.g. process.env = {ENV_VAR: 'val'}; would also trigger the bug, though probably not a good idea in real code :).

[brendankenny]

bisecting leads to #26908.

It seems to occur for any globals, e.g.

// test.js
window.name = 'name';
window.console.log(5); // error TS2339: Property 'console' does not exist on type 'typeof window'.
module.exports = 'anything';

It looks like issue #27057 may be the same issue, but I haven't cloned and run the project given in the example to verify.

[sandersn]

Repros on 3.0 with an expando initializer, like window.name = {}. The bug is more visible now that initializers aren't required to be expandos. The core of the problem is in iniitalizeTypeChecker:

if (!isExternalOrCommonJsModule(file)) {
  mergeSymbolTable(globals, file.locals!);
}

CommonJS modules should still merge with globals, because, in JS, the compiler allows undeclared identifiers to act as namespaces in exactly the way assignment to window works.