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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 2016 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.AbstractPreOrderCallback;
import com.google.javascript.rhino.Node;

/**
 * Checks to make sure the required ES6 externs are present.
 */
final class Es6ExternsCheck extends AbstractPreOrderCallback implements CompilerPass {
  static final DiagnosticType MISSING_ES6_EXTERNS =
      DiagnosticType.error("JSC_MISSING_ES6_EXTERNS",
          "Missing externs definition for Symbol. Did you forget to include the ES6 externs?");

  private final AbstractCompiler compiler;
  private boolean hasSymbolExterns = false;

  Es6ExternsCheck(AbstractCompiler compiler) {
    this.compiler = compiler;
  }

  private boolean hasEs6Syntax(Node root) {
    checkState(root.isRoot());
    for (Node script : root.children()) {
      checkState(script.isScript());
      if (TranspilationPasses.isScriptEs6OrHigher(script)) {
        return true;
      }
    }
    return false;
  }

  @Override
  public void process(Node externs, Node root) {
    NodeTraversal.traverseEs6(compiler, externs, this);
    if (!hasSymbolExterns) {
      if (hasEs6Syntax(root)) {
        compiler.report(JSError.make(MISSING_ES6_EXTERNS));
      }
      VarCheck.createSynthesizedExternVar(compiler, "Symbol");
    }
  }

  @Override
  public boolean shouldTraverse(NodeTraversal t, Node n, Node parent) {
    if (NodeUtil.isFunctionDeclaration(n) && n.getFirstChild().matchesQualifiedName("Symbol")) {
      hasSymbolExterns = true;
    }
    return !hasSymbolExterns;
  }
}