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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. This binary checks for style issues such as incorrect or missing JSDoc usage, and missing goog.require() statements. It does not do more advanced checks such as typechecking.

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/*
 * Copyright 2009 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 com.google.common.annotations.VisibleForTesting;
import com.google.common.base.Preconditions;
import com.google.javascript.jscomp.DefinitionsRemover.Definition;
import com.google.javascript.rhino.IR;
import com.google.javascript.rhino.Node;
import java.util.ArrayList;
import java.util.Collection;
import java.util.List;

/**
 * A compiler pass for optimize function return results.  Currently this
 * pass looks for results that are complete unused and rewrite then to be:
 *   "return x()" -->"x(); return"
 * , but it can easily be
 * expanded to look for use context to avoid unneeded type coercion:
 *   - "return x.toString()" --> "return x"
 *   - "return !!x" --> "return x"
 * @author [email protected] (John Lenz)
 */
class OptimizeReturns
    implements OptimizeCalls.CallGraphCompilerPass, CompilerPass {

  private AbstractCompiler compiler;

  OptimizeReturns(AbstractCompiler compiler) {
    this.compiler = compiler;
  }

  @Override
  @VisibleForTesting
  public void process(Node externs, Node root) {
    DefinitionUseSiteFinder defFinder = new DefinitionUseSiteFinder(compiler);
    defFinder.process(externs, root);
    process(externs, root, defFinder);
  }

  @Override
  public void process(
      Node externs, Node root, DefinitionUseSiteFinder definitions) {
    // Find all function nodes whose callers ignore the return values.
    List toOptimize = new ArrayList<>();
    for (DefinitionSite defSite : definitions.getDefinitionSites()) {
      if (!defSite.inExterns && !callResultsMaybeUsed(definitions, defSite)) {
        toOptimize.add(defSite.definition.getRValue());
      }
    }
    // Optimize the return statements.
    for (Node node : toOptimize) {
      rewriteReturns(definitions, node);
    }
  }

  /**
   * Determines if a function result might be used.  A result might be use if:
   * - Function must is exported.
   * - The definition is never accessed outside a function call context.
   */
  private static boolean callResultsMaybeUsed(
      DefinitionUseSiteFinder defFinder, DefinitionSite definitionSite) {

    Definition definition = definitionSite.definition;

    // Assume non-function definitions results are used.
    Node rValue = definition.getRValue();
    if (rValue == null || !rValue.isFunction()) {
      return true;
    }

    // Be conservative, don't try to optimize any declaration that isn't as
    // simple function declaration or assignment.
    if (!NodeUtil.isSimpleFunctionDeclaration(rValue)) {
      return true;
    }

    if (!defFinder.canModifyDefinition(definition)) {
      return true;
    }

    Collection useSites = defFinder.getUseSites(definition);
    for (UseSite site : useSites) {
      // Assume indirect definitions references use the result
      Node useNodeParent = site.node.getParent();
      if (isCall(site)) {
        Node callNode = useNodeParent;
        Preconditions.checkState(callNode.isCall());
        if (NodeUtil.isExpressionResultUsed(callNode)) {
          return true;
        }
      } else {
        // Allow a standalone name reference.
        //     var a;
        if (!useNodeParent.isVar()) {
          return true;
        }
      }
    }

    // No possible use of the definition result
    return false;
  }

  /**
   * For the supplied function node, rewrite all the return expressions so that:
   *    return foo();
   * becomes:
   *    foo(); return;
   * Useless return will be removed later by the peephole optimization passes.
   */
  private void rewriteReturns(
      final DefinitionUseSiteFinder defFinder, Node fnNode) {
    Preconditions.checkState(fnNode.isFunction());
    final Node body = fnNode.getLastChild();
    NodeUtil.visitPostOrder(
      body,
      new NodeUtil.Visitor() {
        @Override
        public void visit(Node node) {
          if (node.isReturn() && node.hasOneChild()) {
            boolean keepValue = NodeUtil.mayHaveSideEffects(
                node.getFirstChild(), compiler);
            if (!keepValue) {
              defFinder.removeReferences(node.getFirstChild());
            }
            Node result = node.removeFirstChild();
            if (keepValue) {
              node.getParent().addChildBefore(
                IR.exprResult(result).srcref(result), node);
            }
            compiler.reportChangeToEnclosingScope(body);
          }
        }
      },
      new NodeUtil.MatchShallowStatement());
  }

  /**
   * Determines if the name node acts as the function name in a call expression.
   */
  private static boolean isCall(UseSite site) {
    Node node = site.node;
    Node parent = node.getParent();
    return (parent.getFirstChild() == node) && parent.isCall();
  }
}