module: custom --conditions flag option

doc/api/cli.md

@@ -76,6 +76,21 @@ $ node --completion-bash > node_bash_completion
 $ source node_bash_completion
 ```
 
+### `-u`, `--conditions=conditionA,conditionB`
+<!-- YAML
+added: REPLACEME
+-->
+
+> Stability: 1 - Experimental
+
+Enable experimental support for custom conditional exports resolution
+conditions.
+
+Any custom comma-separated string condition names are permitted.
+
+The default Node.js conditions of `"node"`, `"default"`, `"import"` and
+`"require"` will always apply as defined.
+
 ### `--cpu-prof`
 <!-- YAML
 added: v12.0.0
@@ -1232,6 +1247,7 @@ node --require "./a.js" --require "./b.js"
 
 Node.js options that are allowed are:
 <!-- node-options-node start -->
+* `--conditions`, `-u`
 * `--diagnostic-dir`
 * `--disable-proto`
 * `--enable-fips`

doc/api/esm.md

@@ -501,6 +501,19 @@ a nested conditional does not have any mapping it will continue checking
 the remaining conditions of the parent condition. In this way nested
 conditions behave analogously to nested JavaScript `if` statements.
 
+#### Resolving custom conditions
+
+When running Node.js, custom comma-separated conditions can be set with the
+`--conditions` or `-m` flag:
+
+```bash
+node --conditions=development main.js
+```
+
+which would then resolve the `"development"` condition in exports, along with
+the existing `"node"`, `"default"`, `"import"` and `"require"` conditions as
+appropriate.
+
 #### Self-referencing a package using its name
 
 Within a package, the values defined in the package’s

doc/node.1

@@ -78,6 +78,10 @@ Aborting instead of exiting causes a core file to be generated for analysis.
 .It Fl -completion-bash
 Print source-able bash completion script for Node.js.
 .
+.It Fl u , Fl -conditions Ar string
+Use custom conditional exports conditions
+.Ar string
+.
 .It Fl -cpu-prof
 Start the V8 CPU profiler on start up, and write the CPU profile to disk
 before exit. If

lib/internal/modules/cjs/loader.js

@@ -85,6 +85,8 @@ const manifest = getOptionValue('--experimental-policy') ?
   require('internal/process/policy').manifest :
   null;
 const { compileFunction } = internalBinding('contextify');
+const userConditions = getOptionValue('--conditions') &&
+    getOptionValue('--conditions').split(',');
 
 // Whether any user-provided CJS modules had been loaded (executed).
 // Used for internal assertions.
@@ -497,8 +499,12 @@ function applyExports(basePath, expansion) {
   if (typeof pkgExports === 'object') {
     if (ObjectPrototypeHasOwnProperty(pkgExports, mappingKey)) {
       const mapping = pkgExports[mappingKey];
-      return resolveExportsTarget(pathToFileURL(basePath + '/'), mapping, '',
-                                  mappingKey);
+      const resolved = resolveExportsTarget(
+        pathToFileURL(basePath + '/'), mapping, '', mappingKey);
+      if (resolved === null || resolved === undefined)
+        throw new ERR_PACKAGE_PATH_NOT_EXPORTED(
+          basePath, mappingKey);
+      return resolved;
     }
 
     let dirMatch = '';
@@ -515,6 +521,9 @@ function applyExports(basePath, expansion) {
       const subpath = StringPrototypeSlice(mappingKey, dirMatch.length);
       const resolved = resolveExportsTarget(pathToFileURL(basePath + '/'),
                                             mapping, subpath, mappingKey);
+      if (resolved === null || resolved === undefined)
+        throw new ERR_PACKAGE_PATH_NOT_EXPORTED(
+          basePath, mappingKey + subpath);
       // Extension searching for folder exports only
       const rc = stat(resolved);
       if (rc === 0) return resolved;
@@ -602,21 +611,29 @@ function resolveExportsTarget(baseUrl, target, subpath, mappingKey) {
     throw new ERR_INVALID_MODULE_SPECIFIER(mappingKey + subpath, reason);
   } else if (ArrayIsArray(target)) {
     if (target.length === 0)
-      throw new ERR_PACKAGE_PATH_NOT_EXPORTED(
-        baseUrl.pathname, mappingKey + subpath);
+      return null;
     let lastException;
     for (const targetValue of target) {
+      let resolved;
       try {
-        return resolveExportsTarget(baseUrl, targetValue, subpath, mappingKey);
+        resolved = resolveExportsTarget(baseUrl, targetValue, subpath,
+                                        mappingKey);
       } catch (e) {
         lastException = e;
-        if (e.code !== 'ERR_PACKAGE_PATH_NOT_EXPORTED' &&
-            e.code !== 'ERR_INVALID_PACKAGE_TARGET')
+        if (e.code !== 'ERR_INVALID_PACKAGE_TARGET')
           throw e;
       }
+      if (resolved === undefined)
+        continue;
+      if (resolved === null) {
+        lastException = null;
+        continue;
+      }
+      return resolved;
     }
     // Throw last fallback error
-    assert(lastException !== undefined);
+    if (lastException === undefined || lastException === null)
+      return lastException;
     throw lastException;
   } else if (typeof target === 'object' && target !== null) {
     const keys = ObjectKeys(target);
@@ -625,30 +642,17 @@ function resolveExportsTarget(baseUrl, target, subpath, mappingKey) {
           'contain numeric property keys.');
     }
     for (const p of keys) {
-      switch (p) {
-        case 'node':
-        case 'require':
-          try {
-            return resolveExportsTarget(baseUrl, target[p], subpath,
-                                        mappingKey);
-          } catch (e) {
-            if (e.code !== 'ERR_PACKAGE_PATH_NOT_EXPORTED') throw e;
-          }
-          break;
-        case 'default':
-          try {
-            return resolveExportsTarget(baseUrl, target.default, subpath,
-                                        mappingKey);
-          } catch (e) {
-            if (e.code !== 'ERR_PACKAGE_PATH_NOT_EXPORTED') throw e;
-          }
+      if (cjsConditions.has(p) || p === 'default') {
+        const resolved = resolveExportsTarget(baseUrl, target[p], subpath,
+                                              mappingKey);
+        if (resolved === undefined)
+          continue;
+        return resolved;
       }
     }
-    throw new ERR_PACKAGE_PATH_NOT_EXPORTED(
-      baseUrl.pathname, mappingKey + subpath);
+    return undefined;
   } else if (target === null) {
-    throw new ERR_PACKAGE_PATH_NOT_EXPORTED(
-      baseUrl.pathname, mappingKey + subpath);
+    return null;
   }
   throw new ERR_INVALID_PACKAGE_TARGET(baseUrl.pathname, mappingKey, target);
 }
@@ -1005,8 +1009,7 @@ Module._load = function(request, parent, isMain) {
   return module.exports;
 };
 
-// TODO: Use this set when resolving pkg#exports conditions.
-const cjsConditions = new SafeSet(['require', 'node']);
+const cjsConditions = new SafeSet(['require', 'node', ...userConditions]);
 Module._resolveFilename = function(request, parent, isMain, options) {
   if (NativeModule.canBeRequiredByUsers(request)) {
     return request;

lib/internal/modules/esm/resolve.js

@@ -51,7 +51,9 @@ const {
 const { Module: CJSModule } = require('internal/modules/cjs/loader');
 
 const packageJsonReader = require('internal/modules/package_json_reader');
-const DEFAULT_CONDITIONS = ObjectFreeze(['node', 'import']);
+const userConditions = getOptionValue('--conditions') &&
+    getOptionValue('--conditions').split(',');
+const DEFAULT_CONDITIONS = ObjectFreeze(['node', 'import', ...userConditions]);
 const DEFAULT_CONDITIONS_SET = new SafeSet(DEFAULT_CONDITIONS);
 
 
@@ -359,12 +361,9 @@ function isArrayIndex(key) {
 function resolvePackageTarget(
   packageJSONUrl, target, subpath, packageSubpath, base, internal, conditions) {
   if (typeof target === 'string') {
-    const resolved = resolvePackageTargetString(
+    return finalizeResolution(resolvePackageTargetString(
       target, subpath, packageSubpath, packageJSONUrl, base, internal,
-      conditions);
-    if (resolved === null)
-      return null;
-    return finalizeResolution(resolved, base);
+      conditions), base);
   } else if (ArrayIsArray(target)) {
     if (target.length === 0)
       return null;

src/node_options.cc

@@ -281,6 +281,11 @@ DebugOptionsParser::DebugOptionsParser() {
 }
 
 EnvironmentOptionsParser::EnvironmentOptionsParser() {
+  AddOption("--conditions",
+            "set the conditional exports conditions",
+            &EnvironmentOptions::conditions,
+            kAllowedInEnvironment);
+  AddAlias("-u", "--conditions");
   AddOption("--diagnostic-dir",
             "set dir for all output files"
             " (default: current working directory)",

src/node_options.h

@@ -100,6 +100,7 @@ class DebugOptions : public Options {
 class EnvironmentOptions : public Options {
  public:
   bool abort_on_uncaught_exception = false;
+  std::string conditions;
   bool enable_source_maps = false;
   bool experimental_json_modules = false;
   bool experimental_modules = false;

test/es-module/test-esm-custom-exports.mjs

@@ -0,0 +1,10 @@
+// Flags: --conditions=custom-condition,another
+import { mustCall } from '../common/index.mjs';
+import { strictEqual } from 'assert';
+import { requireFixture, importFixture } from '../fixtures/pkgexports.mjs';
+[requireFixture, importFixture].forEach((loadFixture) => {
+  loadFixture('pkgexports/condition')
+    .then(mustCall((actual) => {
+      strictEqual(actual.default, 'from custom condition');
+    }));
+});