Support the node module resolution algorithm for ES6 modules.

Adds a --module_resolution flag to specify whether to use the legacy,
node, or browser module resolution algorithm to locate modules.

Closes #2130

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=145837856

src/com/google/javascript/jscomp/CommandLineRunner.java

@@ -710,10 +710,23 @@ private static class Flags {
         usage = "Rewrite ES6 library calls to use polyfills provided by the compiler's runtime.")
     private boolean rewritePolyfills = true;
 
-    @Option(name = "--print_source_after_each_pass",
-        hidden = true,
-        usage = "Whether to iteratively print resulting JS source per pass.")
-        private boolean printSourceAfterEachPass = false;
+    @Option(
+      name = "--print_source_after_each_pass",
+      hidden = true,
+      usage = "Whether to iteratively print resulting JS source per pass."
+    )
+    private boolean printSourceAfterEachPass = false;
+
+    @Option(
+      name = "--module_resolution",
+      hidden = false,
+      usage =
+          "Specifies how the compiler locates modules. BROWSER requires all module imports "
+              + "to begin with a '.' or '/' and have a file extension. NODE uses the node module "
+              + "rules. LEGACY prepends a '/' to any import not already beginning with a "
+              + "'.' or '/'."
+    )
+    private ModuleLoader.ResolutionMode moduleResolutionMode = ModuleLoader.ResolutionMode.LEGACY;
 
     @Argument
     private List<String> arguments = new ArrayList<>();
@@ -787,7 +800,10 @@ private void parse(List<String> args) throws CmdLineException {
             .putAll(
                 "JS Modules",
                 ImmutableList.of(
-                    "js_module_root", "process_common_js_modules", "transform_amd_modules"))
+                    "js_module_root",
+                    "module_resolution",
+                    "process_common_js_modules",
+                    "transform_amd_modules"))
             .putAll(
                 "Library and Framework Specific",
                 ImmutableList.of(
@@ -1690,6 +1706,7 @@ protected CompilerOptions createOptions() {
     options.setStrictModeInput(flags.strictModeInput);
     options.setEmitUseStrict(flags.emitUseStrict);
     options.setSourceMapIncludeSourcesContent(flags.sourceMapIncludeSourcesContent);
+    options.setModuleResolutionMode(flags.moduleResolutionMode);
 
     return options;
   }

src/com/google/javascript/jscomp/Compiler.java

@@ -1491,9 +1491,15 @@ Node parseInputs() {
           || options.transformAMDToCJSModules
           || options.processCommonJSModules) {
 
-        this.moduleLoader = new ModuleLoader(this, options.moduleRoots, inputs);
-
-        if (options.processCommonJSModules) {
+        this.moduleLoader =
+            new ModuleLoader(
+                this,
+                options.moduleRoots,
+                inputs,
+                ModuleLoader.PathResolver.RELATIVE,
+                options.moduleResolutionMode);
+
+        if (options.moduleResolutionMode == ModuleLoader.ResolutionMode.NODE) {
           this.moduleLoader.setPackageJsonMainEntries(processJsonInputs(inputs));
         }
 

src/com/google/javascript/jscomp/CompilerOptions.java

@@ -1086,10 +1086,13 @@ public void setWrapGoogModulesForWhitespaceOnly(boolean enable) {
    */
   private boolean isStrictModeInput = true;
 
+  /** Which algorithm to use for locating ES6 and CommonJS modules */
+  ModuleLoader.ResolutionMode moduleResolutionMode;
+
   /**
    * Should the compiler print its configuration options to stderr when they are initialized?
    *
-   * <p> Default {@code false}.
+   * <p>Default {@code false}.
    */
   public void setPrintConfig(boolean printConfig) {
     this.printConfig = printConfig;
@@ -1109,6 +1112,9 @@ public CompilerOptions() {
     // Which environment to use
     environment = Environment.BROWSER;
 
+    // Modules
+    moduleResolutionMode = ModuleLoader.ResolutionMode.LEGACY;
+
     // Checks
     skipNonTranspilationPasses = false;
     devMode = DevMode.OFF;
@@ -2648,6 +2654,14 @@ public CompilerOptions setEmitUseStrict(boolean emitUseStrict) {
     return this;
   }
 
+  public ModuleLoader.ResolutionMode getModuleResolutionMode() {
+    return this.moduleResolutionMode;
+  }
+
+  public void setModuleResolutionMode(ModuleLoader.ResolutionMode mode) {
+    this.moduleResolutionMode = mode;
+  }
+
   @Override
   public String toString() {
     String strValue =

src/com/google/javascript/jscomp/DiagnosticGroups.java

@@ -143,8 +143,9 @@ public DiagnosticGroup forName(String name) {
           + "visibility";
 
   public static final DiagnosticGroup COMMON_JS_MODULE_LOAD =
-      DiagnosticGroups.registerGroup(
-          "commonJsModuleLoad", ProcessCommonJSModules.COMMON_JS_MODULE_LOAD_ERROR);
+      DiagnosticGroups.registerGroup("commonJsModuleLoad",
+          ProcessCommonJSModules.SUSPICIOUS_EXPORTS_ASSIGNMENT,
+          ProcessCommonJSModules.UNKNOWN_REQUIRE_ENSURE);
 
   public static final DiagnosticGroup GLOBAL_THIS =
       DiagnosticGroups.registerGroup("globalThis",

src/com/google/javascript/jscomp/ProcessCommonJSModules.java

@@ -43,10 +43,6 @@ public final class ProcessCommonJSModules implements CompilerPass {
   private static final String EXPORTS = "exports";
   private static final String MODULE = "module";
 
-  public static final DiagnosticType COMMON_JS_MODULE_LOAD_ERROR = DiagnosticType.error(
-      "JSC_COMMONJS_MODULE_LOAD_ERROR",
-      "Failed to load module \"{0}\"");
-
   public static final DiagnosticType UNKNOWN_REQUIRE_ENSURE =
       DiagnosticType.warning(
           "JSC_COMMONJS_UNKNOWN_REQUIRE_ENSURE_ERROR", "Unrecognized require.ensure call: {0}");
@@ -356,9 +352,16 @@ public void visit(NodeTraversal t, Node n, Node parent) {
     /** Visit require calls. Emit corresponding goog.require call. */
     private void visitRequireCall(NodeTraversal t, Node require, Node parent) {
       String requireName = require.getSecondChild().getString();
-      ModulePath modulePath = t.getInput().getPath().resolveCommonJsModule(requireName);
+      ModulePath modulePath =
+          t.getInput()
+              .getPath()
+              .resolveJsModule(
+                  requireName,
+                  require.getSourceFileName(),
+                  require.getLineno(),
+                  require.getCharno());
       if (modulePath == null) {
-        compiler.report(t.makeError(require, COMMON_JS_MODULE_LOAD_ERROR, requireName));
+        // The module loader will issue an error
         return;
       }
 
@@ -701,9 +704,16 @@ public void visit(NodeTraversal t, Node n, Node parent) {
      */
     private void visitRequireCall(NodeTraversal t, Node require, Node parent) {
       String requireName = require.getSecondChild().getString();
-      ModulePath modulePath = t.getInput().getPath().resolveCommonJsModule(requireName);
+      ModulePath modulePath =
+          t.getInput()
+              .getPath()
+              .resolveJsModule(
+                  requireName,
+                  require.getSourceFileName(),
+                  require.getLineno(),
+                  require.getCharno());
       if (modulePath == null) {
-        compiler.report(t.makeError(require, COMMON_JS_MODULE_LOAD_ERROR, requireName));
+        // The module loader will issue an error
         return;
       }
 
@@ -1190,7 +1200,11 @@ private String getModuleImportName(NodeTraversal t, Node n) {
             && rValue.getSecondChild().isString()
             && t.getScope().getVar(rValue.getFirstChild().getQualifiedName()) == null) {
           String requireName = rValue.getSecondChild().getString();
-          ModulePath modulePath = t.getInput().getPath().resolveCommonJsModule(requireName);
+          ModulePath modulePath =
+              t.getInput()
+                  .getPath()
+                  .resolveJsModule(
+                      requireName, n.getSourceFileName(), n.getLineno(), n.getCharno());
           if (modulePath == null) {
             return null;
           }
@@ -1227,9 +1241,16 @@ private void fixTypeNode(NodeTraversal t, Node typeNode) {
           }
 
           String moduleName = name.substring(0, endIndex);
-          ModulePath modulePath = t.getInput().getPath().resolveCommonJsModule(moduleName);
+          ModulePath modulePath =
+              t.getInput()
+                  .getPath()
+                  .resolveJsModule(
+                      moduleName,
+                      typeNode.getSourceFileName(),
+                      typeNode.getLineno(),
+                      typeNode.getCharno());
           if (modulePath == null) {
-            t.makeError(typeNode, COMMON_JS_MODULE_LOAD_ERROR, moduleName);
+            // The module loader will issue an error
             return;
           }
 

src/com/google/javascript/jscomp/ProcessEs6Modules.java

@@ -165,7 +165,21 @@ private void visitImport(NodeTraversal t, Node importDecl, Node parent) {
       // These are rewritten to plain namespace object accesses.
       moduleName = importName.substring("goog:".length());
     } else {
-      moduleName = t.getInput().getPath().resolveEs6Module(importName).toModuleName();
+      ModuleLoader.ModulePath modulePath =
+          t.getInput()
+              .getPath()
+              .resolveJsModule(
+                  importName,
+                  importDecl.getSourceFileName(),
+                  importDecl.getLineno(),
+                  importDecl.getCharno());
+      if (modulePath == null) {
+        // The module loader issues an error
+        // Fall back to assuming the module is a file path
+        modulePath = t.getInput().getPath().resolveModuleAsPath(importName);
+      }
+
+      moduleName = modulePath.toModuleName();
     }
 
     for (Node child : importDecl.children()) {
@@ -270,8 +284,18 @@ private void visitExport(NodeTraversal t, Node export, Node parent) {
       parent.addChildBefore(importNode, export);
       visit(t, importNode, parent);
 
-      String moduleName =
-          t.getInput().getPath().resolveEs6Module(moduleIdentifier.getString()).toModuleName();
+      ModuleLoader.ModulePath path =
+          t.getInput()
+              .getPath()
+              .resolveJsModule(
+                  moduleIdentifier.getString(),
+                  export.getSourceFileName(),
+                  export.getLineno(),
+                  export.getCharno());
+      if (path == null) {
+        path = t.getInput().getPath().resolveModuleAsPath(moduleIdentifier.getString());
+      }
+      String moduleName = path.toModuleName();
 
       for (Node exportSpec : export.getFirstChild().children()) {
         String nameFromOtherModule = exportSpec.getFirstChild().getString();
@@ -539,8 +563,20 @@ private void fixTypeNode(NodeTraversal t, Node typeNode) {
           }
 
           String moduleName = name.substring(0, endIndex);
-          String globalModuleName =
-              t.getInput().getPath().resolveEs6Module(moduleName).toModuleName();
+          ModuleLoader.ModulePath path =
+              t.getInput()
+                  .getPath()
+                  .resolveJsModule(
+                      moduleName,
+                      typeNode.getSourceFileName(),
+                      typeNode.getLineno(),
+                      typeNode.getCharno());
+
+          if (path == null) {
+            path = t.getInput().getPath().resolveModuleAsPath(moduleName);
+          }
+          String globalModuleName = path.toModuleName();
+
           typeNode.setString(
               localTypeName == null ? globalModuleName : globalModuleName + localTypeName);
         } else {

src/com/google/javascript/jscomp/deps/JsFileParser.java

@@ -285,7 +285,11 @@ protected boolean parseLine(String line) throws ParseException {
           if (arg.startsWith("goog:")) {
             requires.add(arg.substring(5)); // cut off the "goog:" prefix
           } else {
-            requires.add(file.resolveEs6Module(arg).toModuleName());
+            ModuleLoader.ModulePath path = file.resolveJsModule(arg);
+            if (path == null) {
+              path = file.resolveModuleAsPath(arg);
+            }
+            requires.add(path.toModuleName());
           }
         }
       }