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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 2012 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;

/**
 * Checks for common errors, such as misplaced semicolons:
 *  * if (x); act_now();
 * 
 *  or comparison against NaN:
 *  * if (x === NaN) act();
 * 
 * and generates warnings.
 *
 * @author [email protected] (John Lenz)
 */
final class CheckSuspiciousCode extends AbstractPostOrderCallback {

  static final DiagnosticType SUSPICIOUS_SEMICOLON = DiagnosticType.warning(
      "JSC_SUSPICIOUS_SEMICOLON",
      "If this if/for/while really shouldn''t have a body, use '{}'");

  static final DiagnosticType SUSPICIOUS_COMPARISON_WITH_NAN =
      DiagnosticType.warning(
          "JSC_SUSPICIOUS_NAN",
          "Comparison against NaN is always false. Did you mean isNaN()?");

  static final DiagnosticType SUSPICIOUS_IN_OPERATOR =
      DiagnosticType.warning(
          "JSC_SUSPICIOUS_IN",
          "Use of the \"in\" keyword on non-object types throws an exception.");

  static final DiagnosticType SUSPICIOUS_INSTANCEOF_LEFT_OPERAND =
      DiagnosticType.warning(
          "JSC_SUSPICIOUS_INSTANCEOF_LEFT",
          "\"instanceof\" with left non-object operand is always false.");

  static final DiagnosticType SUSPICIOUS_NEGATED_LEFT_OPERAND_OF_IN_OPERATOR =
      DiagnosticType.warning(
          "JSC_SUSPICIOUS_NEGATED_LEFT_OPERAND_OF_IN_OPERATOR",
          "Suspicious negated left operand of 'in' operator.");

  @Override
  public void visit(NodeTraversal t, Node n, Node parent) {
    checkMissingSemicolon(t, n);
    checkNaN(t, n);
    checkInvalidIn(t, n);
    checkNonObjectInstanceOf(t, n);
    checkNegatedLeftOperandOfInOperator(t, n);
  }

  private void checkMissingSemicolon(NodeTraversal t, Node n) {
    switch (n.getToken()) {
      case IF:
        Node trueCase = n.getSecondChild();
        reportIfWasEmpty(t, trueCase);
        Node elseCase = trueCase.getNext();
        if (elseCase != null) {
          reportIfWasEmpty(t, elseCase);
        }
        break;

      case WHILE:
      case FOR:
      case FOR_IN:
      case FOR_OF:
        reportIfWasEmpty(t, NodeUtil.getLoopCodeBlock(n));
        break;
      default:
        break;
    }
  }

  private static void reportIfWasEmpty(NodeTraversal t, Node block) {
    checkState(block.isNormalBlock());

    // A semicolon is distinguished from a block without children by
    // annotating it with EMPTY_BLOCK.  Blocks without children are
    // usually intentional, especially with loops.
    if (!block.hasChildren() && block.isAddedBlock()) {
        t.getCompiler().report(
            t.makeError(block, SUSPICIOUS_SEMICOLON));
    }
  }

  private void checkNaN(NodeTraversal t, Node n) {
    switch (n.getToken()) {
      case EQ:
      case GE:
      case GT:
      case LE:
      case LT:
      case NE:
      case SHEQ:
      case SHNE:
        reportIfNaN(t, n.getFirstChild());
        reportIfNaN(t, n.getLastChild());
        break;
      default:
        break;
    }
  }

  private static void reportIfNaN(NodeTraversal t, Node n) {
    if (NodeUtil.isNaN(n)) {
      t.getCompiler().report(
          t.makeError(n.getParent(), SUSPICIOUS_COMPARISON_WITH_NAN));
    }
  }

  private void checkInvalidIn(NodeTraversal t, Node n) {
    if (n.isIn()) {
      reportIfNonObject(t, n.getLastChild(), SUSPICIOUS_IN_OPERATOR);
    }
  }

  private void checkNonObjectInstanceOf(NodeTraversal t, Node n) {
    if (n.isInstanceOf()) {
      reportIfNonObject(
          t, n.getFirstChild(), SUSPICIOUS_INSTANCEOF_LEFT_OPERAND);
    }
  }

  private static boolean reportIfNonObject(
      NodeTraversal t, Node n, DiagnosticType diagnosticType) {
    if (n.isAdd() || !NodeUtil.mayBeObject(n)) {
      t.report(n.getParent(), diagnosticType);
      return true;
    }
    return false;
  }

  private void checkNegatedLeftOperandOfInOperator(NodeTraversal t, Node n) {
    if (n.isIn() && n.getFirstChild().isNot()) {
      t.report(n.getFirstChild(), SUSPICIOUS_NEGATED_LEFT_OPERAND_OF_IN_OPERATOR);
    }
  }
}