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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. This binary checks for style issues such as incorrect or missing JSDoc usage, and missing goog.require() statements. It does not do more advanced checks such as typechecking.

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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.lint;

import static com.google.common.base.Preconditions.checkState;

import com.google.common.collect.Multimap;
import com.google.common.collect.MultimapBuilder;
import com.google.common.collect.Sets;
import com.google.javascript.jscomp.AbstractCompiler;
import com.google.javascript.jscomp.CompilerPass;
import com.google.javascript.jscomp.DiagnosticType;
import com.google.javascript.jscomp.JSError;
import com.google.javascript.jscomp.NodeTraversal;
import com.google.javascript.jscomp.NodeTraversal.Callback;
import com.google.javascript.jscomp.NodeUtil;
import com.google.javascript.jscomp.Scope;
import com.google.javascript.jscomp.Var;
import com.google.javascript.rhino.Node;
import com.google.javascript.rhino.Token;
import java.util.HashSet;
import java.util.List;
import java.util.Set;

/** Checks that exports of ES6 modules are not mutated outside of module initialization. */
public final class CheckNoMutatedEs6Exports implements Callback, CompilerPass {

  public static final DiagnosticType MUTATED_EXPORT =
      DiagnosticType.warning(
          "JSC_MUTATED_EXPORT",
          "The name \"{0}\" is exported and should not be mutated outside of module "
              + "initialization. Mutable exports are generally difficult to reason about. You "
              + "can work around this by exporting getter/setter functions, or an object with "
              + "mutable properties instead.");

  private final AbstractCompiler compiler;
  private final Multimap mutatedNames =
      MultimapBuilder.hashKeys().hashSetValues().build();
  private final Set exportedLocalNames = new HashSet<>();

  public CheckNoMutatedEs6Exports(AbstractCompiler compiler) {
    this.compiler = compiler;
  }

  private void checkNoMutations() {
    Set mutatedExports = Sets.intersection(mutatedNames.keySet(), exportedLocalNames);

    for (String mutatedExport : mutatedExports) {
      for (Node mutation : mutatedNames.get(mutatedExport)) {
        compiler.report(JSError.make(mutation, MUTATED_EXPORT, mutatedExport));
      }
    }

    mutatedNames.clear();
    exportedLocalNames.clear();
  }

  @Override
  public void visit(NodeTraversal t, Node n, Node parent) {
    if (n.isModuleBody()) {
      checkNoMutations();
    }
  }

  @Override
  public boolean shouldTraverse(NodeTraversal t, Node n, Node parent) {
    switch (n.getToken()) {
      case SCRIPT:
        return n.getBooleanProp(Node.ES6_MODULE);
      case EXPORT:
        visitExport(n);
        return true;
      case NAME:
        visitName(t, n);
        return true;
      default:
        return true;
    }
  }

  private void visitExport(Node export) {
    if (export.hasOneChild() && export.getFirstChild().getToken() == Token.EXPORT_SPECS) {
      // export {a, b as c};
      for (Node exportSpec : export.getFirstChild().children()) {
        checkState(exportSpec.hasTwoChildren());
        exportedLocalNames.add(exportSpec.getFirstChild().getString());
      }
    } else if (export.hasOneChild() && !export.getBooleanProp(Node.EXPORT_ALL_FROM)) {
      Node declaration = export.getFirstChild();

      if (NodeUtil.isNameDeclaration(declaration)) {
        // export const x = 0;
        // export let a, b, c;
        List lhsNodes = NodeUtil.findLhsNodesInNode(declaration);

        for (Node lhs : lhsNodes) {
          checkState(lhs.isName());
          exportedLocalNames.add(lhs.getString());
        }
      } else if (export.getBooleanProp(Node.EXPORT_DEFAULT)) {
        // export default function() {}
        // export default function foo() {}
        // export default 0;
        // Note: default exports may or may not be declarations.
        if (!declaration.isClass() && !declaration.isFunction()) {
          return;
        }
        Node nameNode = declaration.getFirstChild();
        if (!nameNode.isEmpty() && !nameNode.getString().isEmpty()) {
          exportedLocalNames.add(nameNode.getString());
        }
      } else {
        // export function foo() {}
        // export class Bar {}
        checkState(declaration.isClass() || declaration.isFunction());
        Node nameNode = declaration.getFirstChild();
        exportedLocalNames.add(nameNode.getString());
      }
    }
  }

  private void visitName(NodeTraversal t, Node name) {
    Scope scope = t.getScope();
    if (NodeUtil.isLValue(name) && !scope.getClosestHoistScope().isModuleScope()) {
      Var var = scope.getVar(name.getString());
      if (var != null && var.getScope().isModuleScope()) {
        mutatedNames.put(name.getString(), name);
      }
    }
  }

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