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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 2014 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.lint;

import com.google.javascript.jscomp.AbstractCompiler;
import com.google.javascript.jscomp.DiagnosticType;
import com.google.javascript.jscomp.HotSwapCompilerPass;
import com.google.javascript.jscomp.JSError;
import com.google.javascript.jscomp.NodeTraversal;
import com.google.javascript.jscomp.NodeUtil;
import com.google.javascript.rhino.JSDocInfo;
import com.google.javascript.rhino.Node;

/**
 * Checks when a mutable property is assigned to a prototype. This is
 * generally undesirable because it can lead to the following unexpected
 * situation.
 *
 *  * /** @constructor * /
 * function MyClass() {}
 * MyClass.prototype.prop = [];
 * x = new MyClass;
 * y = new MyClass;
 * x.prop.push(1);
 * console.log(y.prop) // [1]
 * 
 */
public final class CheckPrototypeProperties implements HotSwapCompilerPass, NodeTraversal.Callback {
  public static final DiagnosticType ILLEGAL_PROTOTYPE_MEMBER =
      DiagnosticType.disabled(
          "JSC_ILLEGAL_PROTOTYPE_MEMBER",
          "Prototype property {0} should be a primitive, not an Array or Object.");

  final AbstractCompiler compiler;

  public CheckPrototypeProperties(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, originalRoot, this);
  }

  @Override
  public void visit(NodeTraversal unused, Node n, Node parent) {
    if (NodeUtil.isPrototypePropertyDeclaration(n)) {
      Node assign = n.getFirstChild();
      Node rhs = assign.getLastChild();
      if (rhs.isArrayLit() || rhs.isObjectLit()) {
        JSDocInfo jsDoc = NodeUtil.getBestJSDocInfo(rhs);
        if (jsDoc != null && jsDoc.hasEnumParameterType()) {
          // Don't report for @enum's on the prototype. Sometimes this is necessary, for example,
          // to expose the enum values to an Angular template.
          return;
        }
        String propName = assign.getFirstChild().getLastChild().getString();
        compiler.report(JSError.make(assign, ILLEGAL_PROTOTYPE_MEMBER, propName));
      }
    }
  }

  @Override
  public boolean shouldTraverse(NodeTraversal nodeTraversal, Node n, Node parent) {
    return true;
  }
}