[ES modules] package.json located in root path can't be resolved when checking type field

[vitalets]

• Version: v14.2.0
• Platform: Linux c89b7c439bd7 4.19.76-linuxkit #1 SMP Fri Apr 3 15:53:26 UTC 2020 x86_64 Linux

What steps will reproduce the bug?

Create docker image that generates package.json and index.js in root of filesystem:

FROM node:14-alpine

RUN echo '{ "type": "module" }' > package.json
RUN echo 'import fs from "fs";' > index.js

CMD ["node", "index.js"]

Build and run the container:

docker build -t app .
docker run --rm app

Output:

(node:1) Warning: To load an ES module, set "type": "module" in the package.json or use the .mjs extension.
(Use `node --trace-warnings ...` to show where the warning was created)
/index.js:1
import fs from "fs";
^^^^^^

SyntaxError: Cannot use import statement outside a module
    at Object.compileFunction (vm.js:344:18)
    at wrapSafe (internal/modules/cjs/loader.js:1106:15)
    at Module._compile (internal/modules/cjs/loader.js:1140:27)
    at Object.Module._extensions..js (internal/modules/cjs/loader.js:1196:10)
    at Module.load (internal/modules/cjs/loader.js:1040:32)
    at Function.Module._load (internal/modules/cjs/loader.js:929:14)
    at Function.executeUserEntryPoint [as runMain] (internal/modules/run_main.js:71:12)
    at internal/main/run_main_module.js:17:47

How often does it reproduce? Is there a required condition?

Always.

What is the expected behavior?

index.js should be loaded as es module because the nearest package.json has "type": "module" field.

What do you see instead?

index.js is being loaded as commonjs.

Additional information

If package.json and index.js are generated in some subdirectory - everything works. You can check it with the following image:

FROM node:14-alpine

# add workdir to generate files in /app not in /
WORKDIR app 

RUN echo '{ "type": "module" }' > package.json
RUN echo 'import fs from "fs";' > index.js

CMD ["node", "index.js"]

The reason of such behavior is this line:

(separatorIndex = checkPath.lastIndexOf(path.sep)) > rootSeparatorIndex

When checkPath = '/index.js', both separatorIndex and rootSeparatorIndex are equals to 0 and condition does not pass.

[DerekNonGeneric]

/cc @nodejs/modules-active-members

[guybedford]

Yes this looks like a bug to me, we should support a type at the root. PRs to fix welcome, although understood testing might be an issue!