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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 2006 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.checkState;

import com.google.javascript.jscomp.NodeTraversal.AbstractPostOrderCallback;
import com.google.javascript.rhino.Node;
import com.google.javascript.rhino.Token;
import java.util.ArrayList;
import java.util.HashSet;
import java.util.List;
import java.util.Set;

/**
 * Collapses multiple variable declarations into a single one. i.e the
 * following:
 *
 *  * var a;
 * var b = 1;
 * var c = 2;
 * 
 *
 * becomes:
 *
 * var a, b = 1, c = 2;
 *
 * This reduces the generated code size. More optimizations are possible:
 * Group all variable declarations inside a function into one such variable.
 * declaration block.
 * Re-use variables instead of declaring a new one if they are used for
 * only part of a function.
 *
 * Similarly, also collapses assigns like:
 *
 *  * a = true;
 * b = true;
 * var c = true;
 * 
 *
 * becomes:
 *
 * var c = b = a = true;
 */
class CollapseVariableDeclarations implements CompilerPass {
  /** Reference to JS Compiler */
  private final AbstractCompiler compiler;

  /** Encapsulation of information about a variable declaration collapse */
  private static class Collapse {
    /**
     * Variable declaration that any following var nodes should be
     * collapsed into
     */
    final Node startNode;

    Collapse(Node startNode, Node endNode, Node parent) {
      this.startNode = startNode;
    }
  }

  /** Collapses to do in this pass. */
  private final List collapses = new ArrayList<>();

  /**
   * Nodes we've already looked at for collapsing, so that we don't look at them
   * again (we look ahead when examining what nodes can be collapsed, and the
   * node traversal may give them to us again)
   */
  private final Set nodesToCollapse = new HashSet<>();

  CollapseVariableDeclarations(AbstractCompiler compiler) {
    checkState(!compiler.getLifeCycleStage().isNormalized());
    this.compiler = compiler;
  }

  @Override
  public void process(Node externs, Node root) {
    collapses.clear();
    nodesToCollapse.clear();

    NodeTraversal.traverse(compiler, root, new GatherCollapses());

    if (!collapses.isEmpty()) {
      applyCollapses();
    }
  }

  /**
   * Gathers all of the variable declarations that should be collapsed into one.
   *
   * We do not do the collapsing as we go since node traversal would be affected by the changes
   * we are making to the parse tree.
   */
  private class GatherCollapses extends AbstractPostOrderCallback {

    @Override
    public void visit(NodeTraversal t, Node n, Node parent) {
      // If we've already looked at this node, skip it
      if (nodesToCollapse.contains(n)) {
        return;
      }

      if (!NodeUtil.isNameDeclaration(n)) {
        return;
      }

      // Adjacent VAR children of an IF node are the if and else parts and can't
      // be collapsed
      if (parent.isIf()) {
        return;
      }

      Node varNode = n;
      Token nType = n.getToken();

      // Find variable declarations that follow this one (if any)
      n = n.getNext();
      boolean hasNodesToCollapse = false;

      // we only want to collapse lets with lets, vars with vars, and consts with consts so we
      // check to make sure the declaration types match
      while (n != null && nType == n.getToken()) {

        nodesToCollapse.add(n);
        hasNodesToCollapse = true;

        n = n.getNext();
      }

      if (hasNodesToCollapse) {
        nodesToCollapse.add(varNode);
        collapses.add(new Collapse(varNode, n, parent));
      }
    }
  }

  private void applyCollapses() {
    for (Collapse collapse : collapses) {
      Node var = collapse.startNode;
      compiler.reportChangeToEnclosingScope(var);

      while (var.getNext() != null
          && (var.getNext().getToken() == var.getToken())) {
        Node next = var.getNext().detach();

        // Move all children of the next var node into the first one.
        var.addChildrenToBack(next.removeChildren());
      }
    }
  }
}