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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 2008 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.rhino.IR;
import com.google.javascript.rhino.Node;

/**
 * Prepare the AST before we do any checks or optimizations on it.
 *
 * This pass must run. It should bring the AST into a consistent state,
 * and add annotations where necessary. It should not make any transformations
 * on the tree that would lose source information, since we need that source
 * information for checks.
 */
class PrepareAst implements CompilerPass {

  private final AbstractCompiler compiler;
  private final boolean checkOnly;

  PrepareAst(AbstractCompiler compiler) {
    this(compiler, false);
  }

  PrepareAst(AbstractCompiler compiler, boolean checkOnly) {
    this.compiler = compiler;
    this.checkOnly = checkOnly;
  }

  private void reportChange() {
    if (checkOnly) {
      throw new IllegalStateException("normalizeNodeType constraints violated");
    }
  }

  @Override
  public void process(Node externs, Node root) {
    if (checkOnly) {
      normalizeNodeTypes(root);
    } else {
      // Don't perform "PrepareAnnotations" when doing checks as
      // they currently aren't valid during validity checks.  In particular,
      // they DIRECT_EVAL shouldn't be applied after inlining has been performed.
      if (externs != null) {
        NodeTraversal.traverse(
            compiler, externs, new PrepareAnnotations());
      }
      if (root != null) {
        NodeTraversal.traverse(
            compiler, root, new PrepareAnnotations());
      }
    }
  }

  /**
   * Covert EXPR_VOID to EXPR_RESULT to simplify the rest of the code.
   */
  private void normalizeNodeTypes(Node n) {
    normalizeBlocks(n);

    for (Node child = n.getFirstChild();
         child != null; child = child.getNext()) {
      // This pass is run during the CompilerTestCase validation, so this
      // parent pointer check serves as a more general check.
      checkState(child.getParent() == n);

      normalizeNodeTypes(child);
    }
  }

  /**
   * Add blocks to IF, WHILE, DO, etc.
   */
  private void normalizeBlocks(Node n) {
    if (NodeUtil.isControlStructure(n)
        && !n.isLabel()
        && !n.isSwitch()) {
      for (Node c = n.getFirstChild(); c != null; c = c.getNext()) {
        if (NodeUtil.isControlStructureCodeBlock(n, c) && !c.isBlock()) {
          Node newBlock = IR.block().srcref(n);
          n.replaceChild(c, newBlock);
          newBlock.setIsAddedBlock(true);
          if (!c.isEmpty()) {
            newBlock.addChildrenToFront(c);
          }
          c = newBlock;
          reportChange();
        }
      }
    }
  }

  /**
   * Normalize where annotations appear on the AST. Copies
   * around existing JSDoc annotations as well as internal annotations.
   */
  static class PrepareAnnotations
      extends NodeTraversal.AbstractPostOrderCallback {

    @Override
    public void visit(NodeTraversal t, Node n, Node parent) {
      switch (n.getToken()) {
        case CALL:
        case OPTCHAIN_CALL:
          annotateCalls(n);
          break;
        default:
          break;
      }
    }

    /**
     * There are two types of calls we are interested in calls without explicit
     * "this" values (what we are call "free" calls) and direct call to eval.
     */
    private static void annotateCalls(Node n) {
      checkState(n.isCall() || n.isOptChainCall(), n);

      // Keep track of of the "this" context of a call.  A call without an
      // explicit "this" is a free call.
      Node first = n.getFirstChild();

      // ignore cast nodes.
      while (first.isCast()) {
        first = first.getFirstChild();
      }

      if (!NodeUtil.isNormalOrOptChainGet(first)) {
        n.putBooleanProp(Node.FREE_CALL, true);
      }

      // Keep track of the context in which eval is called. It is important
      // to distinguish between "(0, eval)()" and "eval()".
      if (first.isName() && "eval".equals(first.getString())) {
        first.putBooleanProp(Node.DIRECT_EVAL, true);
      }
    }
  }
}