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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 2020 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.AbstractPostOrderCallback;
import com.google.javascript.jscomp.parsing.parser.FeatureSet;
import com.google.javascript.jscomp.parsing.parser.FeatureSet.Feature;
import com.google.javascript.rhino.Node;

/**
 * Removes trailing commas when transpiling to anything before ECMASCRIPT 2017.
 *
 * {@code
 * function f(a, b,) {}
 * f(1, 2,);
 * let x = new Number(1,);
 * }
 *
 * Becomes
 *
 * {@code
 * function f(a, b) {}
 * f(1, 2);
 * let x = new Number(1);
 * }
 */
final class RemoveTrailingCommaFromParamList implements CompilerPass {
  private static final FeatureSet TRANSPILED_FEATURES =
      FeatureSet.BARE_MINIMUM.with(Feature.TRAILING_COMMA_IN_PARAM_LIST);

  private final AbstractCompiler compiler;

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

  private static class RemoveComma extends AbstractPostOrderCallback {
    @Override
    public void visit(NodeTraversal t, Node n, Node parent) {
      // Optional chaining should be removed before this pass runs
      checkState(!n.isOptChainCall());
      if (n.hasTrailingComma() && (n.isParamList() || n.isCall() || n.isNew())) {
        n.setTrailingComma(false);
        // We do not report this as a code change, because trailing commas are not considered when
        // evaluating the equality of nodes.
      }
    }
  }

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