specify import file specifier resolution proposal

Author: guybedford

doc/api/esm.md

@@ -55,10 +55,6 @@ property:
 
 ## Notable differences between `import` and `require`
 
-### Only Support for .mjs
-
-ESM must have the `.mjs` extension.
-
 ### Mandatory file extensions
 
 You must provide a file extension when using the `import` keyword.
@@ -157,37 +153,132 @@ The resolver has the following properties:
 
 ### Resolver Algorithm
 
-The algorithm to resolve an ES module specifier is provided through
-_ESM_RESOLVE_:
+The algorithm to load an ES module specifier is given through the
+**ESM_RESOLVE** method below. It returns the resolved URL for a 
+module specifier relative to a parentURL, in addition to the unique module
+format for that resolved URL given by the **ESM_FORMAT** routine.
+
+In the following algorithms, all subroutine errors are propogated as errors
+of these top-level routines.
 
-**ESM_RESOLVE**(_specifier_, _parentURL_)
-> 1. Let _resolvedURL_ be _undefined_.
-> 1. If _specifier_ is a valid URL then,
+#### ESM_RESOLVE(_specifier_, _parentURL_)
+> 1. Let _resolvedURL_ be *undefined*.
+> 1. If _specifier_ is a valid URL, then
 >    1. Set _resolvedURL_ to the result of parsing and reserializing
 >       _specifier_ as a URL.
-> 1. Otherwise, if _specifier_ starts with _"/"_, _"./"_ or _"../"_ then,
+> 1. Otherwise, if _specifier_ starts with _"/"_, then
+>    1. Throw an _Invalid Specifier_ error.
+> 1. Otherwise, if _specifier_ starts with _"./"_ or _"../"_, then
 >    1. Set _resolvedURL_ to the URL resolution of _specifier_ relative to
 >       _parentURL_.
 > 1. Otherwise,
 >    1. Note: _specifier_ is now a bare specifier.
 >    1. Set _resolvedURL_ the result of
 >       **PACKAGE_RESOLVE**(_specifier_, _parentURL_).
-> 1. If the file at _resolvedURL_ does not exist then,
+> 1. If the file at _resolvedURL_ does not exist, then
 >    1. Throw a _Module Not Found_ error.
-> 1. Return _resolvedURL_.
-
-**PACKAGE_RESOLVE**(_packageSpecifier_, _parentURL_)
-> 1. Assert: _packageSpecifier_ is a bare specifier.
-> 1. If _packageSpecifier_ is a Node.js builtin module then,
+> 1. Let _format_ be the result of **ESM_FORMAT**(_url_).
+> 1. Return _{ resolvedURL, format }_.
+
+PACKAGE_RESOLVE(_packageSpecifier_, _parentURL_)
+> 1. Let _packageName_ be *undefined*.
+> 1. Let _packagePath_ be *undefined*.
+> 1. If _packageSpecifier_ does not start with _"@"_, then
+>    1. If _packageSpecifier_ is an empty string, then
+>       1. Throw an _Invalid Package Name_ error.
+>    1. Set _packageName_ to the substring of _packageSpecifier_ until the
+>       first _"/"_ separator or the end of the string.
+> 1. If _packageSpecifier_ starts with _"@"_, then
+>    1. If _packageSpecifier_ does not contain a _"/"_ separator, then
+>       1. Throw an _Invalid Package Name_ error.
+>    1. Set _packageName_ to the substring of _packageSpecifier_
+>       until the second _"/"_ separator or the end of the string.
+> 1. Let _packagePath_ be the substring of _packageSpecifier_ from the
+>    position at the length of _packageName_ plus one, if any.
+> 1. Assert: _packageName_ is a valid package name or scoped package name.
+> 1. Assert: _packagePath_ is either empty, or a path without a leading
+>    separator.
+> 1. Note: Further package name validations can be added here.
+> 1. If _packagePath_ is empty and _packageName_ is a Node.js builtin
+>    module, then
 >    1. Return the string _"node:"_ concatenated with _packageSpecifier_.
-> 1. While _parentURL_ contains a non-empty _pathname_,
+> 1. While _parentURL_ is not the file system root,
 >    1. Set _parentURL_ to the parent folder URL of _parentURL_.
 >    1. Let _packageURL_ be the URL resolution of the string concatenation of
->       _parentURL_, _"/node_modules/"_ and _"packageSpecifier"_.
->    1. If the file at _packageURL_ exists then,
->       1. Return _packageURL_.
+>       _parentURL_, _"/node_modules/"_ and _packageSpecifier_.
+>    1. If the folder at _packageURL_ does not exist, then
+>       1. Note: This check can be optimized out where possible in
+>          implementation.
+>       1. Set _parentURL_ to the parent URL path of _parentURL_.
+>       1. Continue the next loop iteration.
+>    1. If _packagePath_ is empty, then
+>       1. Let _url_ be the result of **PACKAGE_MAIN_RESOLVE**(_packageURL_).
+>       1. If _url_ is *null*, then
+>          1. Throw a _Module Not Found_ error.
+>       1. Return _url_.
+>    1. Otherwise,
+>       1. Return the URL resolution of _packagePath_ in _packageURL_.
 > 1. Throw a _Module Not Found_ error.
 
+PACKAGE_MAIN_RESOLVE(_packageURL_)
+> 1. Let _pjsonURL_ be the URL of the file _"package.json"_ within the parent
+     path _packageURL_.
+> 1. If the file at _pjsonURL_ exists, then
+>    1. Let _pjson_ be the result of **READ_JSON_FILE**(_pjsonURL_).
+>    1. If **HAS_ESM_PROPERTIES**(_pjson_) is *true*, then
+>       1. Let _mainURL_ be the result applying the legacy
+>          **LOAD_AS_DIRECTORY** CommonJS resolver to _packageURL_, returning
+>          *undefined* for no resolution.
+>       1. If _mainURL_ is not *undefined* and **ESM_FORMAT**(_mainURL_) is not
+>          equal to _"cjs"_, then
+>          1. Throw a _"Invalid Module Format"_ error.
+>       1. Return _mainURL_.
+> 1. _Note: ESM main yet to be implemented here._
+> 1. Return *null*.
+
+#### ESM_FORMAT(_url_)
+> 1. Assert: _url_ corresponds to an existing file.
+> 1. Let _pjsonURL_ be the result of **READ_PACKAGE_BOUNDARY**(_url_).
+> 1. Let _pjson_ be *undefined*.
+> 1. If _pjsonURL_ is not *null*, then
+>    1. Set _pjson_ to the result of **READ_JSON_FILE**(_pjsonURL_).
+> 1. If _pjsonURL_ is *null* or **HAS_ESM_PROPERTIES**(_pjson_) is *true*, then
+>    1. If _url_ does not end in _".js"_ or _".mjs"_ then,
+>       1. Throw an _Unkonwn Module Format_ error.
+>    1. Return _"esm"_.
+> 1. Otherwise,
+>    1. If _url_ does not end in _".js"_ then,
+>       1. Throw an _Unknown Module Format_ error.
+>    1. Return _"cjs"_.
+
+> 1. If **HAS_ESM_PROPERTIES**(_pjson_) is *true*, then
+>    1. Return _"esm"_.
+> 1. Return _"cjs"_.
+
+READ_PACKAGE_BOUNDARY(_url_)
+> 1. Let _boundaryURL_ be the URL resolution of _"package.json"_ relative to
+>    _url_.
+> 1. While _boundaryURL_ is not the file system root,
+>    1. If the file at _boundaryURL_ exists, then
+>       1. Return _boundaryURL_.
+>    1. Set _boundaryURL_ to the URL resolution of _"../package.json"_ relative
+>       to _boundaryURL_.
+> 1. Return *null*.
+
+READ_JSON_FILE(_url_)
+> 1. If the file at _url_ does not parse as valid JSON, then
+>    1. Throw an _Invalid Package Configuration_ error.
+> 1. Let _pjson_ be the parsed JSON source of the file at _url_.
+> 1. Return _pjson_.
+
+HAS_ESM_PROPERTIES(_pjson_)
+> 1. Note: To be specified.
+
+_ESM properties_ in a package.json file are yet to be specified.
+The current possible specifications for this are the
+[_"exports"_](https://github.com/jkrems/proposal-pkg-exports)
+or [_"mode"_](https://github.com/nodejs/node/pull/18392) flag.
+
 [Node.js EP for ES Modules]: https://github.com/nodejs/node-eps/blob/master/002-es-modules.md
 [`module.createRequireFromPath()`]: modules.html#modules_module_createrequirefrompath_filename
 [ESM Minimal Kernel]: https://github.com/nodejs/modules/blob/master/doc/plan-for-new-modules-implementation.md