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Closure Compiler is a JavaScript optimizing compiler. It parses your JavaScript, analyzes it, removes dead code and rewrites and minimizes what's left. It also checks syntax, variable references, and types, and warns about common JavaScript pitfalls. It is used in many of Google's JavaScript apps, including Gmail, Google Web Search, Google Maps, and Google Docs.

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/*
 * Copyright 2017 The Closure Compiler Authors.
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *     http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */
package com.google.javascript.jscomp;

import static com.google.common.base.Preconditions.checkArgument;

import com.google.javascript.jscomp.NodeTraversal.AbstractPreOrderCallback;
import com.google.javascript.rhino.Node;
import com.google.javascript.rhino.Token;

/** Rewrites a script which was imported as a module into an ES6 module. */
public final class Es6RewriteScriptsToModules extends AbstractPreOrderCallback
    implements CompilerPass {
  private final AbstractCompiler compiler;

  /**
   * Creates a new Es6RewriteModules instance which can be used to rewrite ES6 modules to a
   * concatenable form.
   */
  public Es6RewriteScriptsToModules(AbstractCompiler compiler) {
    this.compiler = compiler;
  }

  /**
   * Force rewriting of a script into an ES6 module, such as for imported files that contain no
   * "import" or "export" statements.
   */
  void forceToEs6Module(Node root) {
    if (Es6RewriteModules.isEs6ModuleRoot(root)) {
      return;
    }
    Node moduleNode = new Node(Token.MODULE_BODY).srcref(root);
    moduleNode.addChildrenToBack(root.removeChildren());
    root.addChildToBack(moduleNode);
    compiler.reportChangeToChangeScope(root);
  }

  @Override
  public void process(Node externs, Node root) {
    for (Node file = root.getFirstChild(); file != null; file = file.getNext()) {
      checkArgument(file.isScript());
      NodeTraversal.traverse(compiler, file, this);
    }
  }

  @Override
  public boolean shouldTraverse(NodeTraversal nodeTraversal, Node n, Node parent) {
    if (n.isScript()) {
      CompilerInput.ModuleType moduleType = compiler.getInput(n.getInputId()).getJsModuleType();

      if (moduleType == CompilerInput.ModuleType.IMPORTED_SCRIPT) {
        forceToEs6Module(n);
      }
    }

    return false;
  }
}