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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 2019 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.jscomp.NodeTraversal.AbstractPostOrderCallback;
import com.google.javascript.jscomp.parsing.parser.FeatureSet;
import com.google.javascript.jscomp.parsing.parser.FeatureSet.Feature;
import com.google.javascript.rhino.Node;

/**
 * Transpiles catch statements with no bindings by adding an unused binding.
 *
 * {@code
 * try {
 *   stuff();
 * } catch {
 *   onError();
 * }
 * }
 *
 * Becomes
 *
 * {@code
 * try {
 *   stuff();
 * } catch ($jscomp$unused$catch) {
 *   onError();
 * }
 * }
 */
final class RewriteCatchWithNoBinding implements HotSwapCompilerPass {
  private static final FeatureSet TRANSPILED_FEATURES =
      FeatureSet.BARE_MINIMUM.with(Feature.OPTIONAL_CATCH_BINDING);
  private static final String BINDING_NAME = "$jscomp$unused$catch";

  private final AbstractCompiler compiler;
  private final AstFactory astFactory;

  RewriteCatchWithNoBinding(AbstractCompiler compiler) {
    this.compiler = compiler;
    this.astFactory = compiler.createAstFactory();
  }

  private class AddBindings extends AbstractPostOrderCallback {
    @Override
    public void visit(NodeTraversal t, Node n, Node parent) {
      if (!n.isCatch() || !n.getFirstChild().isEmpty()) {
        return;
      }

      Node name = astFactory.createNameWithUnknownType(BINDING_NAME);
      n.getFirstChild().replaceWith(name.srcrefTree(n.getFirstChild()));
      t.reportCodeChange();
    }
  }

  @Override
  public void hotSwapScript(Node scriptRoot, Node originalRoot) {
    TranspilationPasses.hotSwapTranspile(
        compiler, scriptRoot, TRANSPILED_FEATURES, new AddBindings());
    TranspilationPasses.maybeMarkFeaturesAsTranspiledAway(compiler, TRANSPILED_FEATURES);
  }

  @Override
  public void process(Node externs, Node root) {
    TranspilationPasses.processTranspile(compiler, root, TRANSPILED_FEATURES, new AddBindings());
    TranspilationPasses.maybeMarkFeaturesAsTranspiledAway(compiler, TRANSPILED_FEATURES);
  }
}