Import from JSON data URL doesn't work

[evanw]

• Version: v15.11.0
• Platform: Darwin Evans-MacBook-Pro.local 19.6.0 Darwin Kernel Version 19.6.0: Tue Jan 12 22:13:05 PST 2021; root:xnu-6153.141.16~1/RELEASE_X86_64 x86_64
• Subsystem: ?

What steps will reproduce the bug?

To reproduce, run this code in an ECMAScript module:

import 'data:text/javascript,console.log("hello!");';
import _ from 'data:application/json,"world!"';

The code will crash immediately. This code is taken verbatim from node's documentation: https://nodejs.org/docs/latest/api/esm.html#esm_data_imports. The relevant part of the documentation is pasted below for reference:

data: Imports

Added in: v12.10.0

data: URLs are supported for importing with the following MIME types:

• text/javascript for ES Modules
• application/json for JSON
• application/wasm for Wasm

data: URLs only resolve Bare specifiers for builtin modules and Absolute specifiers. Resolving Relative specifiers does not work because data: is not a special scheme. For example, attempting to load ./foo from data:text/javascript,import "./foo"; fails to resolve because there is no concept of relative resolution for data: URLs. An example of a data: URLs being used is:

import 'data:text/javascript,console.log("hello!");';
import _ from 'data:application/json,"world!"';

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

Always.

What is the expected behavior?

The snippet should print hello! and exit. Presumably if you add console.log(_) it should print world! as well.

What do you see instead?

node:internal/process/esm_loader:74
    internalBinding('errors').triggerUncaughtException(
                              ^

TypeError [ERR_INVALID_RETURN_PROPERTY_VALUE]: Expected string to be returned for the "format" from the "loader getFormat" function but got type object.
    at new NodeError (node:internal/errors:329:5)
    at Loader.getFormat (node:internal/modules/esm/loader:111:13)
    at async Loader.getModuleJob (node:internal/modules/esm/loader:231:20)
    at async ModuleWrap.<anonymous> (node:internal/modules/esm/module_job:59:21)
    at async Promise.all (index 1)
    at async link (node:internal/modules/esm/module_job:64:9) {
  code: 'ERR_INVALID_RETURN_PROPERTY_VALUE'
}

This error message goes away when the application/json import is removed.

Additional information

I am logging this because I am trying to understand if this is supposed to work. If not, the documentation probably just needs to be updated. But if it is supposed to work, it would be good to know if the example code needs to be updated. For context I'm the main developer of the esbuild bundler and I'm trying to have it match node's behavior here, so I need to know what the behavior is supposed to be.

[targos]

It's expected that this code doesn't work without the --experimental-json-modules flag. However, a crash like this instead of a clear error message is probably a bug.

[benjamingr]

@nodejs/modules also #37375