[New] add moduleSystem option to select import or require conditions for exports resolution

Previously, `getCategoryInfo` was called with a hardcoded `'require'` module system.
This adds an `opts.moduleSystem` option (`'import'` or `'require'`, defaulting to `'require'`) that is passed through instead, allowing callers to resolve using import conditions (e.g. `['import', 'node', 'default']`) rather than require conditions (e.g. `['node', 'require', 'default']`).

Author: ljharb

lib/async.js

@@ -165,6 +165,13 @@ module.exports = function resolve(x, options, callback) {
     }
     var packageIterator = opts.packageIterator;
 
+    if (typeof opts.moduleSystem !== 'undefined' && opts.moduleSystem !== 'require' && opts.moduleSystem !== 'import') {
+        var msErr = new $TypeError('`moduleSystem` must be `\'require\'` or `\'import\'`.');
+        return process.nextTick(function () {
+            cb(msErr);
+        });
+    }
+
     var extensions = opts.extensions || ['.js'];
     var includeCoreModules = opts.includeCoreModules !== false;
     var basedir = opts.basedir || path.dirname(caller());
@@ -237,7 +244,7 @@ module.exports = function resolve(x, options, callback) {
 
             // If package has exports field, resolve via exports
             if (typeof pkg.exports !== 'undefined') {
-                var categoryInfo = getCategoryInfo(exportsCategory, 'require');
+                var categoryInfo = getCategoryInfo(exportsCategory, opts.moduleSystem || 'require');
                 var conditions = opts.conditions || categoryInfo.conditions;
                 var resolved;
                 try {
@@ -542,7 +549,7 @@ module.exports = function resolve(x, options, callback) {
 
     function loadNodeModulesWithExports(x, start, done) {
         var parsed = parsePackageSpecifier(x);
-        var categoryInfo = getCategoryInfo(exportsCategory, 'require');
+        var categoryInfo = getCategoryInfo(exportsCategory, opts.moduleSystem || 'require');
         var conditions = opts.conditions || categoryInfo.conditions;
 
         var thunk = function () { return getPackageCandidates(parsed.name, start, opts); };

lib/sync.js

@@ -145,6 +145,10 @@ module.exports = function resolveSync(x, options) {
     }
     var packageIterator = opts.packageIterator;
 
+    if (typeof opts.moduleSystem !== 'undefined' && opts.moduleSystem !== 'require' && opts.moduleSystem !== 'import') {
+        throw new $TypeError('`moduleSystem` must be `\'require\'` or `\'import\'`.');
+    }
+
     var extensions = opts.extensions || ['.js'];
     var includeCoreModules = opts.includeCoreModules !== false;
     var basedir = opts.basedir || path.dirname(caller());
@@ -215,7 +219,7 @@ module.exports = function resolveSync(x, options) {
 
         // If package has exports field, resolve via exports
         if (typeof pkg.exports !== 'undefined') {
-            var categoryInfo = getCategoryInfo(exportsCategory, 'require');
+            var categoryInfo = getCategoryInfo(exportsCategory, opts.moduleSystem || 'require');
             var conditions = opts.conditions || categoryInfo.conditions;
             var resolved = resolveExports(pkg.exports, parsed.subpath, conditions, categoryInfo.flags);
             if (resolved) {
@@ -346,7 +350,7 @@ module.exports = function resolveSync(x, options) {
 
     function loadNodeModulesWithExportsSync(x, start) {
         var parsed = parsePackageSpecifier(x);
-        var categoryInfo = getCategoryInfo(exportsCategory, 'require');
+        var categoryInfo = getCategoryInfo(exportsCategory, opts.moduleSystem || 'require');
         var conditions = opts.conditions || categoryInfo.conditions;
 
         // Get candidate directories for the package name

readme.markdown

@@ -126,8 +126,10 @@ This also enables **self-reference** support, allowing a package to import itsel
 
 **Note:** `exportsCategory` and `engines` are mutually exclusive - only one can be specified.
 
+* opts.moduleSystem - either `'require'` or `'import'`. Determines which module system conditions are used when resolving the `exports` field. When `'require'` (the default), the conditions include `require`; when `'import'`, the conditions include `import` instead. This option only has effect when `exportsCategory` or `engines` is also set. For example, given a package with `exports: { ".": { "import": "./esm.js", "require": "./cjs.js" } }`, resolving with `moduleSystem: 'import'` returns `esm.js`, while `moduleSystem: 'require'` (or omitting it) returns `cjs.js`.
+
 * opts.conditions - an array of condition strings (e.g. `['require', 'node']`) to use when resolving the `exports` field.
-If specified, this overrides the conditions that would otherwise be derived from the category.
+If specified, this overrides the conditions that would otherwise be derived from the category (including those from `moduleSystem`).
 This option only has effect when `exportsCategory` or `engines` is also set.
 
 default `opts` values:
@@ -245,7 +247,9 @@ This is the way Node resolves dependencies when executed with the [--preserve-sy
 
 **Note:** `exportsCategory`, `enginesRange`, and `engines` are mutually exclusive - only one can be specified.
 
-* opts.conditions - an array of condition strings (e.g. `['require', 'node']`) to use when resolving the `exports` field. If specified, this overrides the conditions that would otherwise be derived from the category. This option only has effect when one of `exportsCategory`, `enginesRange`, or `engines` is also set.
+* opts.moduleSystem - either `'require'` or `'import'`. Determines which module system conditions are used when resolving the `exports` field. When `'require'` (the default), the conditions include `require`; when `'import'`, the conditions include `import` instead. This option only has effect when `exportsCategory` or `engines` is also set. For example, given a package with `exports: { ".": { "import": "./esm.js", "require": "./cjs.js" } }`, resolving with `moduleSystem: 'import'` returns `esm.js`, while `moduleSystem: 'require'` (or omitting it) returns `cjs.js`.
+
+* opts.conditions - an array of condition strings (e.g. `['require', 'node']`) to use when resolving the `exports` field. If specified, this overrides the conditions that would otherwise be derived from the category (including those from `moduleSystem`). This option only has effect when one of `exportsCategory`, `enginesRange`, or `engines` is also set.
 
 default `opts` values: