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9 changes: 6 additions & 3 deletions lib/internal/modules/esm/get_format.js
Original file line number Diff line number Diff line change
Expand Up @@ -55,15 +55,18 @@ const protocolHandlers = ObjectAssign(ObjectCreate(null), {
},
'file:'(parsed, url) {
const ext = extname(parsed.pathname);
let format;
let format = null;

if (ext === '.js') {
format = getPackageType(parsed.href) === 'module' ? 'module' : 'commonjs';
} else {
format = extensionFormatMap[ext];
}
if (!format) {
if (experimentalSpecifierResolution === 'node') {
if (
experimentalSpecifierResolution === 'node' &&
legacyExtensionFormatMap[ext]
) {
process.emitWarning(
'The Node.js specifier resolution in ESM is experimental.',
'ExperimentalWarning');
Expand All @@ -73,7 +76,7 @@ const protocolHandlers = ObjectAssign(ObjectCreate(null), {
}
}

return format || null;
return format;
},
'node:'() { return 'builtin'; },
});
Expand Down
14 changes: 14 additions & 0 deletions test/es-module/test-esm-get-format.js
Original file line number Diff line number Diff line change
@@ -0,0 +1,14 @@
'use strict';
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You need to add a flag for the test to work.

Suggested change
'use strict';
// Flags: --expose-internals
'use strict';

require('../common');
const assert = require('assert');
const { defaultGetFormat } = require('internal/modules/esm/get_format');

{
const url = new URL('file://example.com/foo/bar.js');
assert.strictEqual(defaultGetFormat(url), 'commonjs');
}

{
const url = new URL('file://example.com/foo/bar.whatever');
assert.throws(() => defaultGetFormat(url), { name: 'TypeError', message: /Unknown file extension whatever/ });
}