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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 2015 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.AbstractPostOrderCallback;
import com.google.javascript.jscomp.parsing.parser.FeatureSet.Feature;
import com.google.javascript.rhino.IR;
import com.google.javascript.rhino.Node;

/** An optimization that does peephole optimizations of ES6 code. */
class SubstituteEs6Syntax extends AbstractPostOrderCallback implements CompilerPass {

  private final AbstractCompiler compiler;
  private boolean objectLiteralShorthandWasAdded = false;

  public SubstituteEs6Syntax(AbstractCompiler compiler) {
    this.compiler = compiler;
  }

  @Override
  public void process(Node externs, Node root) {
    NodeTraversal.traverse(compiler, root, this);
  }

  @Override
  public void visit(NodeTraversal t, Node n, Node parent) {
    switch (n.getToken()) {
      case FUNCTION:
        if (n.isArrowFunction()) {
          maybeSimplifyArrowFunctionBody(n, n.getLastChild());
        }
        break;
      case STRING_KEY:
        if (n.getFirstChild().isName() && n.getFirstChild().getString().equals(n.getString())) {
          n.setShorthandProperty(true);
          objectLiteralShorthandWasAdded = true;
        }
        break;
      case SCRIPT:
        if (objectLiteralShorthandWasAdded) {
          NodeUtil.addFeatureToScript(n, Feature.EXTENDED_OBJECT_LITERALS, compiler);
          objectLiteralShorthandWasAdded = false; // false for the next script
        }
        break;
      default:
        break;
    }
  }

  /**
   * If possible, replace functions of the form ()=>{ return x; } with ()=>x
   */
  private void maybeSimplifyArrowFunctionBody(Node arrowFunction, Node body) {
    checkArgument(arrowFunction.isArrowFunction());
    if (!body.isBlock() || !body.hasOneChild() || !body.getFirstChild().isReturn()) {
      return;
    }
    Node returnValue = body.getFirstChild().removeFirstChild();
    Node replacement = returnValue != null ? returnValue : IR.name("undefined");
    body.replaceWith(replacement);
    compiler.reportChangeToEnclosingScope(replacement);
  }
}