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 +153,128 @@ 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.
+
+The _"esm"_ format is returned for an ECMAScript Module, while the
+_"legacy"_ format is used to indicate loading through the legacy
+CommonJS loader. Additional formats such as _"wasm"_ or _"addon"_ can be
+extended in future updates.
 
-**ESM_RESOLVE**(_specifier_, _parentURL_)
-> 1. Let _resolvedURL_ be _undefined_.
-> 1. If _specifier_ is a valid URL then,
+In the following algorithms, all subroutine errors are propogated as errors
+of these top-level routines.
+
+_isMain_ is **true** when resolving the Node.js application entry point.
+
+**ESM_RESOLVE(_specifier_, _parentURL_, _isMain_)**
+> 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_, _isMain_).
+> 1. Load _resolvedURL_ as module format, _format_.
+
+PACKAGE_RESOLVE(_packageSpecifier_, _parentURL_)
+> 1. Let _packageName_ be *undefined*.
+> 1. Let _packageSubpath_ be *undefined*.
+> 1. If _packageSpecifier_ is an empty string, then
+>    1. Throw an _Invalid Specifier_ error.
+> 1. If _packageSpecifier_ does not start with _"@"_, then
+>    1. Set _packageName_ to the substring of _packageSpecifier_ until the
+>       first _"/"_ separator or the end of the string.
+> 1. Otherwise,
+>    1. If _packageSpecifier_ does not contain a _"/"_ separator, then
+>       1. Throw an _Invalid Specifier_ error.
+>    1. Set _packageName_ to the substring of _packageSpecifier_
+>       until the second _"/"_ separator or the end of the string.
+> 1. Let _packageSubpath_ 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: _packageSubpath_ is either empty, or a path without a leading
+>    separator.
+> 1. If _packageSubpath_ contains any _"."_ or _".."_ segments or percent
+>    encoded strings for _"/"_ or _"\"_ then,
+>    1. Throw an _Invalid Specifier_ error.
+> 1. If _packageSubpath_ 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. Set _parentURL_ to the parent URL path of _parentURL_.
+>       1. Continue the next loop iteration.
+>    1. Let _pjson_ be the result of **READ_PACKAGE_JSON**(_packageURL_).
+>    1. If _packageSubpath_ is empty, then
+>       1. Return the result of **PACKAGE_MAIN_RESOLVE**(_packageURL_,
+>          _pjson_).
+>    1. Otherwise,
+>       1. Return the URL resolution of _packageSubpath_ in _packageURL_.
 > 1. Throw a _Module Not Found_ error.
 
+PACKAGE_MAIN_RESOLVE(_packageURL_, _pjson_)
+> 1. If _pjson_ is **null**, then
+>    1. Throw a _Module Not Found_ error.
+> 1. If _pjson.main_ is a String, then
+>    1. Let _resolvedMain_ be the concatenation of _packageURL_, "/", and
+>       _pjson.main_.
+>    1. If the file at _resolvedMain_ exists, then
+>       1. Return _resolvedMain_.
+> 1. If _pjson.type_ is equal to _"esm"_, then
+>    1. Throw a _Module Not Found_ error.
+> 1. Let _legacyMainURL_ be the result applying the legacy
+>    **LOAD_AS_DIRECTORY** CommonJS resolver to _packageURL_, throwing a
+>    _Module Not Found_ error for no resolution.
+> 1. If _legacyMainURL_ does not end in _".js"_ then,
+>    1. Throw an _Unsupported File Extension_ error.
+> 1. Return _legacyMainURL_.
+
+**ESM_FORMAT(_url_, _isMain_)**
+> 1. Assert: _url_ corresponds to an existing file.
+> 1. Let _pjson_ be the result of **READ_PACKAGE_BOUNDARY**(_url_).
+> 1. If _pjson_ is **null** and _isMain_ is **true**, then
+>    1. Return _"legacy"_.
+> 1. If _pjson.type_ exists and is _"esm"_, then
+>    1. If _url_ does not end in _".js"_ or _".mjs"_, then
+>       1. Throw an _Unsupported File Extension_ error.
+>    1. Return _"esm"_.
+> 1. Otherwise,
+>    1. If _url_ ends in _".mjs"_, then
+>       1. Return _"esm"_.
+>    1. Otherwise,
+>       1. Return _"legacy"_.
+
+READ_PACKAGE_BOUNDARY(_url_)
+> 1. Let _boundaryURL_ be _url_.
+> 1. While _boundaryURL_ is not the file system root,
+>    1. Let _pjson_ be the result of **READ_PACKAGE_JSON**(_boundaryURL_).
+>    1. If _pjson_ is not **null**, then
+>       1. Return _pjson_.
+>    1. Set _boundaryURL_ to the parent URL of _boundaryURL_.
+> 1. Return **null**.
+
+READ_PACKAGE_JSON(_packageURL_)
+> 1. Let _pjsonURL_ be the resolution of _"package.json"_ within _packageURL_.
+> 1. If the file at _pjsonURL_ does not exist, then
+>    1. Return **null**.
+> 1. If the file at _packageURL_ does not parse as valid JSON, then
+>    1. Throw an _Invalid Package Configuration_ error.
+> 1. Return the parsed JSON source of the file at _pjsonURL_.
+
 [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