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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.javascript.jscomp.parsing.parser.FeatureSet.ES2018;
import static com.google.javascript.jscomp.parsing.parser.FeatureSet.ES6;
import static com.google.javascript.jscomp.parsing.parser.FeatureSet.ES7;
import static com.google.javascript.jscomp.parsing.parser.FeatureSet.ES8;
import static com.google.javascript.jscomp.parsing.parser.FeatureSet.ES_NEXT;

import com.google.javascript.jscomp.Es6RewriteDestructuring.ObjectDestructuringRewriteMode;
import com.google.javascript.jscomp.NodeTraversal.Callback;
import com.google.javascript.jscomp.PassFactory.HotSwapPassFactory;
import com.google.javascript.jscomp.parsing.parser.FeatureSet;
import com.google.javascript.jscomp.parsing.parser.FeatureSet.Feature;
import com.google.javascript.rhino.Node;
import java.util.List;

/**
 * Provides a single place to manage transpilation passes.
 */
public class TranspilationPasses {
  private TranspilationPasses() {}

  public static void addEs6ModulePass(
      List passes,
      final PreprocessorSymbolTable.CachedInstanceFactory preprocessorTableFactory) {
    passes.add(
        new HotSwapPassFactory("es6RewriteModule") {
          @Override
          protected HotSwapCompilerPass create(AbstractCompiler compiler) {
            preprocessorTableFactory.maybeInitialize(compiler);
            return new Es6RewriteModules(
                compiler,
                compiler.getModuleMetadataMap(),
                compiler.getModuleMap(),
                preprocessorTableFactory.getInstanceOrNull());
          }

          @Override
          protected FeatureSet featureSet() {
            return ES_NEXT;
          }
        });
  }

  public static void addPreTypecheckTranspilationPasses(
      List passes, CompilerOptions options) {

    // TODO(bradfordcsmith): Rename this since it isn't just libraries for ES6 features anymore.
    // Inject runtime libraries needed for the transpilation we will have to do.
    passes.add(es6InjectRuntimeLibraries);

  }

  public static void addEs6ModuleToCjsPass(List passes) {
    passes.add(es6RewriteModuleToCjs);
  }

  public static void addEs6RewriteImportPathPass(List passes) {
    passes.add(es6RelativizeImportPaths);
  }

  /** Adds transpilation passes that should run after all checks are done. */
  public static void addPostCheckTranspilationPasses(
      List passes, CompilerOptions options) {
    if (options.needsTranspilationFrom(FeatureSet.ES_NEXT)) {
      passes.add(rewriteCatchWithNoBinding);
    }

    if (options.needsTranspilationFrom(ES2018)) {
      passes.add(rewriteAsyncIteration);
      passes.add(rewriteObjectSpread);
    }

    if (options.needsTranspilationFrom(ES8)) {
      // Trailing commas in parameter lists are flagged as present by the parser,
      // but never actually represented in the AST.
      // The only thing we need to do is mark them as not present in the AST.
      passes.add(
          createFeatureRemovalPass(
              "markTrailingCommasInParameterListsRemoved", Feature.TRAILING_COMMA_IN_PARAM_LIST));
      passes.add(rewriteAsyncFunctions);
    }

    if (options.needsTranspilationFrom(ES7)) {
      passes.add(rewriteExponentialOperator);
    }

    if (options.needsTranspilationFrom(ES6)) {
      // TODO(b/73387406): Move passes here as typechecking & other check passes are updated to cope
      // with the features they transpile and as the passes themselves are updated to propagate type
      // information to the transpiled code.
      // Binary and octal literals are effectively transpiled by the parser.
      // There's no transpilation we can do for the new regexp flags.
      passes.add(
          createFeatureRemovalPass(
              "markEs6FeaturesNotRequiringTranspilationAsRemoved",
              Feature.BINARY_LITERALS,
              Feature.OCTAL_LITERALS,
              Feature.REGEXP_FLAG_U,
              Feature.REGEXP_FLAG_Y));

      passes.add(es6NormalizeShorthandProperties);
      passes.add(es6RewriteClassExtends);
      passes.add(es6ConvertSuper);
      passes.add(es6RenameVariablesInParamLists);
      passes.add(es6SplitVariableDeclarations);
      passes.add(
          getEs6RewriteDestructuring(ObjectDestructuringRewriteMode.REWRITE_ALL_OBJECT_PATTERNS));
      passes.add(es6RewriteArrowFunction);
      passes.add(es6ExtractClasses);
      passes.add(es6RewriteClass);
      passes.add(es6RewriteRestAndSpread);
      passes.add(lateConvertEs6ToEs3);
      passes.add(es6ForOf);
      passes.add(rewriteBlockScopedFunctionDeclaration);
      passes.add(rewriteBlockScopedDeclaration);
      passes.add(rewriteGenerators);
      passes.add(es6ConvertSuperConstructorCalls);
    } else if (options.needsTranspilationOf(Feature.OBJECT_PATTERN_REST)) {
      passes.add(es6RenameVariablesInParamLists);
      passes.add(es6SplitVariableDeclarations);
      passes.add(getEs6RewriteDestructuring(ObjectDestructuringRewriteMode.REWRITE_OBJECT_REST));
    }
  }

  /**
   * Adds the pass to inject ES6 polyfills, which goes after the late ES6 passes.
   */
  public static void addRewritePolyfillPass(List passes) {
    passes.add(rewritePolyfills);
  }


  /** Rewrites ES6 modules */
  private static final PassFactory es6RewriteModuleToCjs =
      new PassFactory("es6RewriteModuleToCjs", true) {
        @Override
        protected CompilerPass create(AbstractCompiler compiler) {
          return new Es6RewriteModulesToCommonJsModules(compiler);
        }

        @Override
        protected FeatureSet featureSet() {
          return FeatureSet.latest();
        }
      };

  /** Rewrites ES6 modules import paths to be browser compliant */
  private static final PassFactory es6RelativizeImportPaths =
      new PassFactory("es6RelativizeImportPaths", true) {
        @Override
        protected CompilerPass create(AbstractCompiler compiler) {
          return new Es6RelativizeImportPaths(compiler);
        }

        @Override
        protected FeatureSet featureSet() {
          return FeatureSet.latest();
        }
      };

  private static final PassFactory rewriteAsyncFunctions =
      new HotSwapPassFactory("rewriteAsyncFunctions") {
        @Override
        protected HotSwapCompilerPass create(final AbstractCompiler compiler) {
          return new RewriteAsyncFunctions.Builder(compiler)
              // If ES6 classes will not be transpiled away later,
              // transpile away property references that use `super` in async functions.
              // See explanation in RewriteAsyncFunctions.
              .rewriteSuperPropertyReferencesWithoutSuper(
                  !compiler.getOptions().needsTranspilationFrom(FeatureSet.ES6))
              .build();
        }

        @Override
        protected FeatureSet featureSet() {
          return ES_NEXT;
        }
      };

  private static final PassFactory rewriteAsyncIteration =
      new HotSwapPassFactory("rewriteAsyncIteration") {
        @Override
        protected HotSwapCompilerPass create(final AbstractCompiler compiler) {
          return new RewriteAsyncIteration.Builder(compiler)
              // If ES6 classes will not be transpiled away later,
              // transpile away property references that use `super` in async iteration.
              // See explanation in RewriteAsyncIteration.
              .rewriteSuperPropertyReferencesWithoutSuper(
                  !compiler.getOptions().needsTranspilationFrom(FeatureSet.ES6))
              .build();
        }

        @Override
        protected FeatureSet featureSet() {
          return ES_NEXT;
        }
      };

  private static final PassFactory rewriteObjectSpread =
      new HotSwapPassFactory("rewriteObjectSpread") {
        @Override
        protected HotSwapCompilerPass create(final AbstractCompiler compiler) {
          return new RewriteObjectSpread(compiler);
        }

        @Override
        protected FeatureSet featureSet() {
          return ES_NEXT;
        }
      };

  private static final PassFactory rewriteCatchWithNoBinding =
      new HotSwapPassFactory("rewriteCatchWithNoBinding") {
        @Override
        protected HotSwapCompilerPass create(final AbstractCompiler compiler) {
          return new RewriteCatchWithNoBinding(compiler);
        }

        @Override
        protected FeatureSet featureSet() {
          return ES_NEXT;
        }
      };

  private static final PassFactory rewriteExponentialOperator =
      new HotSwapPassFactory("rewriteExponentialOperator") {
        @Override
        protected HotSwapCompilerPass create(final AbstractCompiler compiler) {
          return new Es7RewriteExponentialOperator(compiler);
        }

        @Override
        protected FeatureSet featureSet() {
          return ES2018;
        }
      };

  private static final HotSwapPassFactory es6NormalizeShorthandProperties =
      new HotSwapPassFactory("es6NormalizeShorthandProperties") {
        @Override
        protected HotSwapCompilerPass create(AbstractCompiler compiler) {
          return new Es6NormalizeShorthandProperties(compiler);
        }

        @Override
        protected FeatureSet featureSet() {
          return ES2018;
        }
      };

  static final HotSwapPassFactory es6RewriteClassExtends =
      new HotSwapPassFactory(PassNames.ES6_REWRITE_CLASS_EXTENDS) {
        @Override
        protected HotSwapCompilerPass create(AbstractCompiler compiler) {
          return new Es6RewriteClassExtendsExpressions(compiler);
        }

        @Override
        protected FeatureSet featureSet() {
          return ES2018;
        }
      };

  static final HotSwapPassFactory es6ExtractClasses =
      new HotSwapPassFactory(PassNames.ES6_EXTRACT_CLASSES) {
        @Override
        protected HotSwapCompilerPass create(AbstractCompiler compiler) {
          return new Es6ExtractClasses(compiler);
        }

        @Override
        protected FeatureSet featureSet() {
          return ES8;
        }
      };

  static final HotSwapPassFactory es6RewriteClass =
      new HotSwapPassFactory("Es6RewriteClass") {
        @Override
        protected HotSwapCompilerPass create(AbstractCompiler compiler) {
          return new Es6RewriteClass(compiler);
        }

        @Override
        protected FeatureSet featureSet() {
          return ES8;
        }
      };

  static final HotSwapPassFactory getEs6RewriteDestructuring(
      ObjectDestructuringRewriteMode rewriteMode) {
    return new HotSwapPassFactory("Es6RewriteDestructuring") {
      @Override
      protected HotSwapCompilerPass create(final AbstractCompiler compiler) {
        return new Es6RewriteDestructuring.Builder(compiler)
            .setDestructuringRewriteMode(rewriteMode)
            .build();
      }

      @Override
      protected FeatureSet featureSet() {
        return ES2018;
      }
    };
  }

  static final HotSwapPassFactory es6RenameVariablesInParamLists =
      new HotSwapPassFactory("Es6RenameVariablesInParamLists") {
        @Override
        protected HotSwapCompilerPass create(final AbstractCompiler compiler) {
          return new Es6RenameVariablesInParamLists(compiler);
        }

        @Override
        protected FeatureSet featureSet() {
          return ES2018;
        }
      };

  static final HotSwapPassFactory es6RewriteArrowFunction =
      new HotSwapPassFactory("Es6RewriteArrowFunction") {
        @Override
        protected HotSwapCompilerPass create(final AbstractCompiler compiler) {
          return new Es6RewriteArrowFunction(compiler);
        }

        @Override
        protected FeatureSet featureSet() {
          return ES8;
        }
      };

  static final HotSwapPassFactory rewritePolyfills =
      new HotSwapPassFactory("RewritePolyfills") {
        @Override
        protected HotSwapCompilerPass create(final AbstractCompiler compiler) {
          return new RewritePolyfills(compiler);
        }

        @Override
        protected FeatureSet featureSet() {
          return FeatureSet.latest();
        }
      };

  static final HotSwapPassFactory es6SplitVariableDeclarations =
      new HotSwapPassFactory("Es6SplitVariableDeclarations") {
        @Override
        protected HotSwapCompilerPass create(final AbstractCompiler compiler) {
          return new Es6SplitVariableDeclarations(compiler);
        }

        @Override
        protected FeatureSet featureSet() {
          return FeatureSet.latest();
        }
      };

  static final HotSwapPassFactory es6ConvertSuperConstructorCalls =
      new HotSwapPassFactory("es6ConvertSuperConstructorCalls") {
        @Override
        protected HotSwapCompilerPass create(final AbstractCompiler compiler) {
          return new Es6ConvertSuperConstructorCalls(compiler);
        }

        @Override
        protected FeatureSet featureSet() {
          return ES8;
        }
      };

  static final HotSwapPassFactory es6ConvertSuper =
      new HotSwapPassFactory("es6ConvertSuper") {
        @Override
        protected HotSwapCompilerPass create(final AbstractCompiler compiler) {
          return new Es6ConvertSuper(compiler);
        }

        @Override
        protected FeatureSet featureSet() {
          return ES2018;
        }
      };

  /** Injects runtime library code needed for transpiled ES6 code. */
  static final HotSwapPassFactory es6InjectRuntimeLibraries =
      new HotSwapPassFactory("es6InjectRuntimeLibraries") {
        @Override
        protected HotSwapCompilerPass create(final AbstractCompiler compiler) {
          return new Es6InjectRuntimeLibraries(compiler);
        }

        @Override
        protected FeatureSet featureSet() {
          return ES_NEXT;
        }
      };

  /** Transpiles REST parameters and SPREAD in both array literals and function calls. */
  static final HotSwapPassFactory es6RewriteRestAndSpread =
      new HotSwapPassFactory("es6RewriteRestAndSpread") {
        @Override
        protected HotSwapCompilerPass create(final AbstractCompiler compiler) {
          return new Es6RewriteRestAndSpread(compiler);
        }

        @Override
        protected FeatureSet featureSet() {
          return FeatureSet.latest();
        }
      };

  /**
   * Does the main ES6 to ES3 conversion. There are a few other passes which run before this one, to
   * convert constructs which are not converted by this pass. This pass can run after NTI
   */
  static final HotSwapPassFactory lateConvertEs6ToEs3 =
      new HotSwapPassFactory("lateConvertEs6") {
        @Override
        protected HotSwapCompilerPass create(final AbstractCompiler compiler) {
          return new LateEs6ToEs3Converter(compiler);
        }

        @Override
        protected FeatureSet featureSet() {
          return FeatureSet.latest();
        }
      };

  static final HotSwapPassFactory es6ForOf =
      new HotSwapPassFactory("es6ForOf") {
        @Override
        protected HotSwapCompilerPass create(final AbstractCompiler compiler) {
          return new Es6ForOfConverter(compiler);
        }

        @Override
        protected FeatureSet featureSet() {
          return ES8;
        }
      };

  static final HotSwapPassFactory rewriteBlockScopedFunctionDeclaration =
      new HotSwapPassFactory("Es6RewriteBlockScopedFunctionDeclaration") {
    @Override
    protected HotSwapCompilerPass create(final AbstractCompiler compiler) {
      return new Es6RewriteBlockScopedFunctionDeclaration(compiler);
    }

    @Override
    protected FeatureSet featureSet() {
      return ES8;
    }
  };

  static final HotSwapPassFactory rewriteBlockScopedDeclaration =
      new HotSwapPassFactory("Es6RewriteBlockScopedDeclaration") {
    @Override
    protected HotSwapCompilerPass create(final AbstractCompiler compiler) {
      return new Es6RewriteBlockScopedDeclaration(compiler);
    }

    @Override
    protected FeatureSet featureSet() {
      return ES8;
    }
  };

  static final HotSwapPassFactory rewriteGenerators =
      new HotSwapPassFactory("rewriteGenerators") {
    @Override
    protected HotSwapCompilerPass create(final AbstractCompiler compiler) {
      return new Es6RewriteGenerators(compiler);
    }

    @Override
    protected FeatureSet featureSet() {
      return ES8;
    }
  };

  /**
   * @param script The SCRIPT node representing a JS file
   * @return If the file has any features which are part of ES6 or higher but not part of ES5.
   */
  static boolean isScriptEs6OrHigher(Node script) {
    FeatureSet features = NodeUtil.getFeatureSetOfScript(script);
    return features != null && !FeatureSet.ES5.contains(features);
  }

  /**
   * @param script The SCRIPT node representing a JS file
   * @return If the file has any features not in {@code supportedFeatures}
   */
  static boolean doesScriptHaveUnsupportedFeatures(Node script, FeatureSet supportedFeatures) {
    FeatureSet features = NodeUtil.getFeatureSetOfScript(script);
    return features != null && !supportedFeatures.contains(features);
  }

  /**
   * Process transpilations if the input language needs transpilation from certain features, on any
   * JS file that has features not present in the compiler's output language mode.
   *
   * @param compiler An AbstractCompiler
   * @param combinedRoot The combined root for all JS files.
   * @param featureSet The features which this pass helps transpile.
   * @param callbacks The callbacks that should be invoked if a file has ES6 features.
   */
  static void processTranspile(
      AbstractCompiler compiler, Node combinedRoot, FeatureSet featureSet, Callback... callbacks) {
    if (compiler.getOptions().needsTranspilationFrom(featureSet)) {
      FeatureSet languageOutFeatures = compiler.getOptions().getOutputFeatureSet();
      for (Node singleRoot : combinedRoot.children()) {

        // Only run the transpilation if this file has features not in the compiler's target output
        // language. For example, if this file is purely ES6 and the output language is ES6, don't
        // run any transpilation passes on it.
        // TODO(lharker): We could save time by being more selective about what files we transpile.
        // e.g. if a file has async functions but not `**`, don't run `**` transpilation on it.
        // Right now we know what features were in a file at parse time, but not what features were
        // added to that file by other transpilation passes.
        if (doesScriptHaveUnsupportedFeatures(singleRoot, languageOutFeatures)) {
          for (Callback callback : callbacks) {
            singleRoot.putBooleanProp(Node.TRANSPILED, true);
            NodeTraversal.traverse(compiler, singleRoot, callback);
          }
        }
      }
    }
  }

  /**
   * Hot-swap ES6+ transpilations if the input language needs transpilation from certain features,
   * on any JS file that has features not present in the compiler's output language mode.
   *
   * @param compiler An AbstractCompiler
   * @param scriptRoot The SCRIPT root for the JS file.
   * @param featureSet The features which this pass helps transpile.
   * @param callbacks The callbacks that should be invoked if the file has ES6 features.
   */
  static void hotSwapTranspile(
      AbstractCompiler compiler, Node scriptRoot, FeatureSet featureSet, Callback... callbacks) {
    if (compiler.getOptions().needsTranspilationFrom(featureSet)) {
      FeatureSet languageOutFeatures = compiler.getOptions().getOutputFeatureSet();
      if (doesScriptHaveUnsupportedFeatures(scriptRoot, languageOutFeatures)) {
        for (Callback callback : callbacks) {
          scriptRoot.putBooleanProp(Node.TRANSPILED, true);
          NodeTraversal.traverse(compiler, scriptRoot, callback);
        }
      }
    }
  }

  static void maybeMarkFeaturesAsTranspiledAway(
      AbstractCompiler compiler, FeatureSet transpiledFeatures) {
    // We don't bother to do this if the compiler has halting errors, which avoids unnecessary
    // warnings from AstValidator warning that the features are still there.
    if (!compiler.hasHaltingErrors()) {
      compiler.setFeatureSet(compiler.getFeatureSet().without(transpiledFeatures));
    }
  }

  static void maybeMarkFeaturesAsTranspiledAway(
      AbstractCompiler compiler, Feature transpiledFeature, Feature... moreTranspiledFeatures) {
    if (!compiler.hasHaltingErrors()) {
      compiler.setFeatureSet(
          compiler.getFeatureSet().without(transpiledFeature, moreTranspiledFeatures));
    }
  }

  /**
   * Returns a pass that just removes features from the AST FeatureSet.
   *
   * Doing this indicates that the AST no longer contains uses of the features, or that they are
   * no longer of concern for some other reason.
   */
  private static HotSwapPassFactory createFeatureRemovalPass(
      String passName, final Feature featureToRemove, final Feature... moreFeaturesToRemove) {
    return new HotSwapPassFactory(passName) {
      @Override
      protected HotSwapCompilerPass create(final AbstractCompiler compiler) {
        return new HotSwapCompilerPass() {
          @Override
          public void hotSwapScript(Node scriptRoot, Node originalRoot) {
            maybeMarkFeaturesAsTranspiledAway(compiler, featureToRemove, moreFeaturesToRemove);
          }

          @Override
          public void process(Node externs, Node root) {
            maybeMarkFeaturesAsTranspiledAway(compiler, featureToRemove, moreFeaturesToRemove);
          }
        };
      }

      @Override
      protected FeatureSet featureSet() {
        return FeatureSet.latest();
      }
    };
  }
}