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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 2017 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.bundle;

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

import com.google.common.annotations.GwtIncompatible;
import com.google.common.collect.ImmutableList;
import com.google.errorprone.annotations.Immutable;
import com.google.javascript.jscomp.CheckLevel;
import com.google.javascript.jscomp.Compiler;
import com.google.javascript.jscomp.CompilerOptions;
import com.google.javascript.jscomp.CompilerOptions.LanguageMode;
import com.google.javascript.jscomp.DiagnosticGroup;
import com.google.javascript.jscomp.DiagnosticType;
import com.google.javascript.jscomp.ErrorFormat;
import com.google.javascript.jscomp.JSError;
import com.google.javascript.jscomp.MessageFormatter;
import com.google.javascript.jscomp.PropertyRenamingPolicy;
import com.google.javascript.jscomp.Result;
import com.google.javascript.jscomp.SourceFile;
import com.google.javascript.jscomp.VariableRenamingPolicy;
import java.io.IOException;
import java.nio.file.Path;
import java.util.Optional;

/**
 * A source transformer base class. May also include a runtime that needs to
 * be shipped with the final bundle.
 */
@GwtIncompatible
@Immutable
public abstract class CompilerBasedTransformer implements Source.Transformer {

  private final CompilerSupplier compilerSupplier;

  public CompilerBasedTransformer(CompilerSupplier compilerSupplier) {
    this.compilerSupplier = checkNotNull(compilerSupplier);
  }

  public abstract Optional getRuntime();
  public abstract String getTranformationName();

  @Override
  public Source transform(Source input) {
    CompileResult result = compilerSupplier.compile(input.path(), input.code());
    if (!result.errors.isEmpty()) {
      // TODO(sdh): how to handle this?  Currently we throw an ISE with the message,
      // but this may not be the most appropriate option.  It might make sense to
      // add console.log() statements to any JS that comes out, particularly for
      // warnings.
      MessageFormatter formatter = ErrorFormat.SOURCELESS.toFormatter(null, false);
      StringBuilder message =
          new StringBuilder().append(getTranformationName()).append(" failed.\n");
      for (JSError error : result.errors) {
        message.append(formatter.formatError(error));
      }
      throw new IllegalStateException(message.toString());
    }
    if (!result.transformed) {
      return input;
    }
    Source.Builder builder = input.toBuilder()
        .setCode(result.source)
        .setSourceMap(result.sourceMap);
    if (getRuntime().isPresent()) {
        builder.addRuntime(compilerSupplier.runtime(getRuntime().get()));
    }
    return builder.build();
  }

  /**
   * Wraps the Compiler into a more relevant interface, making it easy to test the
   * CompilerBasedTransformer without depending on implementation details of the Compiler itself.
   * Also works around the fact that the Compiler is not thread-safe (since we may do multiple
   * transpiles concurrently), so we supply a fresh instance each time when we're in single-file
   * mode.
   */
  @Immutable
  public static class CompilerSupplier {

    public CompileResult compile(Path path, String code) {
      Compiler compiler = compiler();
      Result result =
          compiler.compile(EXTERNS, SourceFile.fromCode(path.toString(), code), options());
      String source = compiler.toSource();
      StringBuilder sourceMap = new StringBuilder();
      if (result.sourceMap != null) {
        try {
          result.sourceMap.appendTo(sourceMap, path.toString());
        } catch (IOException e) {
          // impossible, and not a big deal even if it did happen.
        }
      }
      boolean transformed = transformed(result);
      return new CompileResult(
          source, result.errors, transformed, transformed ? sourceMap.toString() : "");
    }

    public boolean transformed(Result result) {
      return result.transpiledFiles;
    }

    public String runtime(String library) {
      Compiler compiler = compiler();
      CompilerOptions options = options();
      options.setForceLibraryInjection(ImmutableList.of(library));
      compiler.compile(EXTERNS, EMPTY, options);
      return compiler.toSource();
    }

    protected Compiler compiler() {
      return new Compiler();
    }

    protected CompilerOptions options() {
      CompilerOptions options = new CompilerOptions();
      setOptions(options);
      return options;
    }

    protected void setOptions(CompilerOptions options) {
        // TODO(sdh): It would be nice to allow people to output code in
        // strict mode.  But currently we swallow all the input language
        // strictness checks, and there are various tests that are never
        // compiled and so are broken when we output 'use strict'.  We
        // could consider adding some sort of logging/warning/error in
        // cases where the input was not strict, though there could still
        // be semantic differences even if syntax is strict.  Possibly
        // the first step would be to allow the option of outputting strict
        // and then change the default and see what breaks.  b/33005948
        options.setLanguageOut(LanguageMode.ECMASCRIPT5);
        options.setQuoteKeywordProperties(true);
        options.setSkipNonTranspilationPasses(true);
        options.setVariableRenaming(VariableRenamingPolicy.OFF);
        options.setPropertyRenaming(PropertyRenamingPolicy.OFF);
        options.setWrapGoogModulesForWhitespaceOnly(false);
        options.setPrettyPrint(true);
        options.setSourceMapOutputPath("/dev/null");
        options.setSourceMapIncludeSourcesContent(true);
        options.setWarningLevel(ES5_WARNINGS, CheckLevel.OFF);
    }

    protected static final SourceFile EXTERNS =
        SourceFile.fromCode("externs.js", "function Symbol() {}");
    protected static final SourceFile EMPTY = SourceFile.fromCode("empty.js", "");
    protected static final DiagnosticGroup ES5_WARNINGS =
        new DiagnosticGroup(DiagnosticType.error("JSC_CANNOT_CONVERT", ""));
  }

  /** The source together with the additional compilation results. */
  public static class CompileResult {
    public final String source;
    public final ImmutableList errors;
    public final boolean transformed;
    public final String sourceMap;

    public CompileResult(
        String source, ImmutableList errors, boolean transformed, String sourceMap) {
      this.source = checkNotNull(source);
      this.errors = checkNotNull(errors);
      this.transformed = transformed;
      this.sourceMap = checkNotNull(sourceMap);
    }
  }
}