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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.transpile;

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

import com.google.common.base.Strings;
import com.google.common.collect.ImmutableList;
import com.google.common.collect.ImmutableMap;
import com.google.javascript.jscomp.CheckLevel;
import com.google.javascript.jscomp.Compiler;
import com.google.javascript.jscomp.CompilerOptions;
import com.google.javascript.jscomp.CompilerOptions.Es6ModuleTranspilation;
import com.google.javascript.jscomp.CompilerOptions.LanguageMode;
import com.google.javascript.jscomp.DiagnosticGroups;
import com.google.javascript.jscomp.PropertyRenamingPolicy;
import com.google.javascript.jscomp.Result;
import com.google.javascript.jscomp.SourceFile;
import com.google.javascript.jscomp.SourceMap;
import com.google.javascript.jscomp.VariableRenamingPolicy;
import com.google.javascript.jscomp.annotations.LegacySetFeatureSetCaller;
import com.google.javascript.jscomp.bundle.TranspilationException;
import com.google.javascript.jscomp.deps.ModuleLoader;
import com.google.javascript.jscomp.deps.ModuleLoader.PathEscaper;
import com.google.javascript.jscomp.deps.ModuleLoader.ResolutionMode;
import com.google.javascript.jscomp.parsing.parser.FeatureSet;
import com.google.javascript.jscomp.parsing.parser.FeatureSet.Feature;
import java.io.IOException;
import java.net.URI;
import java.net.URISyntaxException;

/** Basic Transpiler implementation for outputting ES5 code. */
public final class BaseTranspiler implements Transpiler {

  private final CompilerSupplier compilerSupplier;
  private final String runtimeLibraryName;

  public BaseTranspiler(CompilerSupplier compilerSupplier, String runtimeLibraryName) {
    this.compilerSupplier = checkNotNull(compilerSupplier);
    this.runtimeLibraryName = checkNotNull(runtimeLibraryName);
  }

  @Override
  public TranspileResult transpile(URI path, String code) {
    CompileResult result = compilerSupplier.compile(path, code);
    if (!result.transpiled) {
      return new TranspileResult(path, code, code, "");
    }
    return new TranspileResult(path, code, result.source, result.sourceMap);
  }

  @Override
  public String runtime() {
    StringBuilder sb = new StringBuilder();
    if (!Strings.isNullOrEmpty(runtimeLibraryName)) {
      sb.append(compilerSupplier.runtime(runtimeLibraryName));
    }
    sb.append(compilerSupplier.runtime("modules"));
    return sb.toString();
  }

  public static final BaseTranspiler LATEST_TRANSPILER = to(FeatureSet.latest(), "");

  public static final BaseTranspiler ES5_TRANSPILER = to(LanguageMode.ECMASCRIPT5.toFeatureSet());

  public static final BaseTranspiler to(FeatureSet featureSet, String runtime) {
    return new BaseTranspiler(new CompilerSupplier(featureSet), runtime);
  }

  public static final BaseTranspiler to(FeatureSet featureSet) {
    return to(featureSet, "es6_runtime");
  }

  /**
   * Wraps the Compiler into a more relevant interface, making it easy to test the Transpiler
   * 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.
   */
  public static class CompilerSupplier {
    protected final ResolutionMode moduleResolution;
    protected final ImmutableList moduleRoots;
    protected final ImmutableMap prefixReplacements;
    protected final FeatureSet outputFeatureSet;

    public CompilerSupplier() {
      this(LanguageMode.ECMASCRIPT5.toFeatureSet());
    }

    public CompilerSupplier(FeatureSet outputFeatureSet) {
      // Use the default resolution mode
      this(
          outputFeatureSet,
          new CompilerOptions().getModuleResolutionMode(),
          ImmutableList.of(),
          ImmutableMap.of());
    }

    /**
     * Accepts commonly overridden options for ES6 modules to avoid needed to subclass.
     *
     * @param moduleResolution module resolution for resolving import paths
     * @param prefixReplacements prefix replacements for when moduleResolution is {@link
     *     ModuleLoader.ResolutionMode#BROWSER_WITH_TRANSFORMED_PREFIXES}
     */
    public CompilerSupplier(
        FeatureSet outputFeatureSet,
        ResolutionMode moduleResolution,
        ImmutableList moduleRoots,
        ImmutableMap prefixReplacements) {
      this.outputFeatureSet = outputFeatureSet;
      this.moduleResolution = moduleResolution;
      this.moduleRoots = moduleRoots;
      this.prefixReplacements = prefixReplacements;
    }

    public CompileResult compile(URI path, String code) {
      Compiler compiler = compiler();
      Result result =
          compiler.compile(
              createTrivialExterns(), 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 transpiled = !result.transpiledFiles.isEmpty();
      if (!result.errors.isEmpty()) {
        throw new TranspilationException(compiler, result.errors, result.warnings);
      }
      return new CompileResult(source, transpiled, transpiled ? sourceMap.toString() : "");
    }

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

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

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

    @LegacySetFeatureSetCaller
    protected void setOptions(CompilerOptions options) {
      options.setLanguageIn(LanguageMode.ECMASCRIPT_NEXT);
      options.legacySetOutputFeatureSet(outputFeatureSet.without(Feature.MODULES));
      options.setEmitUseStrict(false);
      options.setQuoteKeywordProperties(true);
      options.setSkipNonTranspilationPasses(true);
      options.setVariableRenaming(VariableRenamingPolicy.OFF);
      options.setPropertyRenaming(PropertyRenamingPolicy.OFF);
      options.setWrapGoogModulesForWhitespaceOnly(false);
      options.setPrettyPrint(true);
      options.setWarningLevel(DiagnosticGroups.NON_STANDARD_JSDOC, CheckLevel.OFF);
      options.setEs6ModuleTranspilation(Es6ModuleTranspilation.TO_COMMON_JS_LIKE_MODULES);
      options.setModuleResolutionMode(moduleResolution);
      options.setModuleRoots(moduleRoots);
      options.setBrowserResolverPrefixReplacements(prefixReplacements);

      // Transpiler often used in isolation, so references to other files will never exist.
      options.setWarningLevel(DiagnosticGroups.MODULE_LOAD, CheckLevel.OFF);

      // Don't escape module paths when bundling in the event paths are URLs.
      options.setPathEscaper(PathEscaper.CANONICALIZE_ONLY);
      options.setParseInlineSourceMaps(true);
      options.setApplyInputSourceMaps(true);
      options.setSourceMapOutputPath("/dev/null");
      options.setSourceMapIncludeSourcesContent(true);
      // Make sourcemaps use absolute paths, so that the path is not duplicated if a build tool adds
      // a sourceurl. Exception: if the location has a scheme (like http:) then leave the path
      // intact. This makes this usable from web servers.
      options.setSourceMapLocationMappings(
          ImmutableList.of(
              (location) -> {
                try {
                  if (new URI(location).getScheme() != null) {
                    return location;
                  }
                } catch (URISyntaxException e) {
                  // Swallow, return the absolute version below.
                }
                return new SourceMap.PrefixLocationMapping("", "/").map(location);
              }));
    }

    protected SourceFile createTrivialExterns() {
      return SourceFile.fromCode("externs.js", "function Symbol() {}");
    }

    protected SourceFile createEmptySource() {
      return SourceFile.fromCode("empty.js", "");
    }
  }

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

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