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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.lint;

import com.google.javascript.jscomp.AbstractCompiler;
import com.google.javascript.jscomp.CompilerPass;
import com.google.javascript.jscomp.DiagnosticType;
import com.google.javascript.jscomp.NodeTraversal;
import com.google.javascript.rhino.Node;
import java.util.HashMap;
import java.util.Map;

/** Miscellaneous checks for style in ES6 modules. */
public final class CheckEs6Modules implements NodeTraversal.Callback, CompilerPass {

  public static final DiagnosticType DUPLICATE_IMPORT =
      DiagnosticType.warning(
          "JSC_DUPLICATE_IMPORT", "The module \"{0}\" has already been imported at {1}, {2}.");

  public static final DiagnosticType NO_DEFAULT_EXPORT =
      DiagnosticType.warning(
          "JSC_DEFAULT_EXPORT",
          "Do not use the default export. There is no way to force consistent naming when "
              + "imported.");

  private final AbstractCompiler compiler;
  private final Map importSpecifiers = new HashMap<>();

  public CheckEs6Modules(AbstractCompiler compiler) {
    this.compiler = compiler;
  }

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

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

  private void visitImport(NodeTraversal t, Node importNode) {
    String specifier = importNode.getLastChild().getString();
    Node duplicateImport = importSpecifiers.putIfAbsent(specifier, importNode);
    if (duplicateImport != null) {
      t.report(
          importNode,
          DUPLICATE_IMPORT,
          specifier,
          "" + duplicateImport.getLineno(),
          "" + duplicateImport.getCharno());
    }
  }

  private void visitExport(NodeTraversal t, Node export) {
    if (export.getBooleanProp(Node.EXPORT_DEFAULT)) {
      t.report(export, NO_DEFAULT_EXPORT);
    }
  }

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