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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 2018 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.javascript.rhino.JSDocInfo;
import com.google.javascript.rhino.JSDocInfo.Visibility;
import com.google.javascript.rhino.Node;
import com.google.javascript.rhino.jstype.JSType;
import java.util.ArrayList;
import java.util.HashSet;
import java.util.List;
import java.util.Set;

/**
 * This pass looks for properties that are not modified and ensures they use the @const annotation.
 * Can run with or without typechecking but will find more if typechecking is enabled.
 */
class CheckConstPrivateProperties extends NodeTraversal.AbstractPostOrderCallback
    implements HotSwapCompilerPass {

  static final DiagnosticType MISSING_CONST_PROPERTY =
      DiagnosticType.disabled(
          "JSC_MISSING_CONST_PROPERTY",
          "Private property {0} is never modified, use the @const annotation");

  private final AbstractCompiler compiler;
  private final List candidates = new ArrayList<>();
  private final Set modified = new HashSet<>();

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

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

  @Override
  public void hotSwapScript(Node scriptRoot, Node originalRoot) {
    NodeTraversal.traverse(compiler, scriptRoot, this);
  }

  /** Reports the property definitions that should use the @const annotation. */
  private void reportMissingConst(NodeTraversal t) {
    for (Node n : candidates) {
      String propName = n.getLastChild().getString();
      if (!modified.contains(propName)) {
        t.report(n, MISSING_CONST_PROPERTY, propName);
      }
    }
    candidates.clear();
    modified.clear();
  }

  @Override
  public void visit(NodeTraversal t, Node n, Node parent) {
    switch (n.getToken()) {
      case SCRIPT:
        // Exiting the script, report any non-const privates not modified in the file.
        reportMissingConst(t);
        break;

      case GETELEM:
      case GETPROP:
        // GETELEM is anytime a property is accessed on an object using the '["${key}"]' syntax.
        // GETPROP is anytime a property is accessed on an object using the '.' syntax.
        Node lastChild = n.getLastChild();
        if (!lastChild.isString()) {
          return;
        }
        String propName = lastChild.getString();

        // Only consider non-const @private class properties as candidates
        if (isCandidatePropertyDefinition(n)) {
          candidates.add(n);
        } else if (isModificationOp(n)) {
          // Mark any other modification operation as a modified property, to deal with lambdas, etc
          modified.add(propName);
        }
        break;

      default:
        break;
    }
  }

  /**
   * @return Whether the given node is a @private property declaration that is not marked constant.
   */
  private boolean isCandidatePropertyDefinition(Node n) {
    if (!NodeUtil.isLhsOfAssign(n)) {
      return false;
    }

    Node target = n.getFirstChild();
    // Check whether the given property access is on 'this' or a static property on a class.
    if (!(target.isThis() || isConstructor(target))) {
      return false;
    }

    JSDocInfo info = NodeUtil.getBestJSDocInfo(n);
    return info != null
        && info.getVisibility() == Visibility.PRIVATE
        && !info.isConstant()
        && !info.hasTypedefType()
        && !info.hasEnumParameterType()
        && !info.isInterface()
        && !isFunctionProperty(n);
  }

  /** @return Whether the given property declaration is assigned to a function. */
  private boolean isFunctionProperty(Node n) {
    // TODO(dylandavidson): getAssignedValue does not support GETELEM.
    if (n.isGetElem()) {
      return false;
    }
    Node assignedValue = NodeUtil.getAssignedValue(n);
    return assignedValue != null && assignedValue.isFunction();
  }

  /**
   * @return Whether the given property is modified in any way (assignment, increment/decrement, or
   *     'delete' property).
   */
  private boolean isModificationOp(Node n) {
    Node parent = n.getParent();

    if (n != parent.getFirstChild()) {
      return false;
    }

    return NodeUtil.isAssignmentOp(parent)
        || parent.isInc()
        || parent.isDec()
        || parent.isDelProp();
  }

  private boolean isConstructor(Node n) {
    // If type checking is enabled (not just a per-file lint check),
    // we can check constructor properties too. But it isn't required.
    JSType type = n.getJSType();
    return type != null && (type.isConstructor() || type.isInterface());
  }
}