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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 2018 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.deps;

import com.google.common.collect.ImmutableList;
import com.google.common.collect.ImmutableMap;
import com.google.common.collect.ImmutableSet;
import com.google.javascript.jscomp.ErrorHandler;
import com.google.javascript.jscomp.deps.ModuleLoader.ModuleResolverFactory;
import com.google.javascript.jscomp.deps.ModuleLoader.PathEscaper;
import java.util.HashMap;
import java.util.Map;
import java.util.Map.Entry;
import org.jspecify.nullness.Nullable;

/**
 * Resolution algorithm for Webpack. Modules are located by a map of numeric ids to module paths.
 *
 * As the compiler normally locates modules by path string, webpack numeric ids are converted to
 * strings.
 */
public class WebpackModuleResolver extends NodeModuleResolver {
  private final ImmutableMap modulesById;

  /**
   * Uses a lookup map provided by webpack to locate modules from a numeric id used during import
   */
  public static final class Factory implements ModuleResolverFactory {
    private final Map lookupMap;

    public Factory(Map lookupMap) {
      this.lookupMap = lookupMap;
    }

    @Override
    public ModuleResolver create(
        ImmutableSet modulePaths,
        ImmutableList moduleRootPaths,
        ErrorHandler errorHandler,
        PathEscaper pathEscaper) {
      Map normalizedPathsById = new HashMap<>();
      for (Entry moduleEntry : lookupMap.entrySet()) {
        String canonicalizedPath =
            ModuleLoader.normalize(pathEscaper.escape(moduleEntry.getValue()), moduleRootPaths);
        if (ModuleLoader.isAmbiguousIdentifier(canonicalizedPath)) {
          canonicalizedPath = ModuleLoader.MODULE_SLASH + canonicalizedPath;
        }
        normalizedPathsById.put(moduleEntry.getKey(), canonicalizedPath);
      }
      return new WebpackModuleResolver(
          modulePaths, moduleRootPaths, normalizedPathsById, errorHandler, pathEscaper);
    }
  }

  public WebpackModuleResolver(
      ImmutableSet modulePaths,
      ImmutableList moduleRootPaths,
      Map modulesById,
      ErrorHandler errorHandler,
      PathEscaper pathEscaper) {
    super(
        modulePaths,
        moduleRootPaths,
        /* packageJsonMainEntries= */ null,
        errorHandler,
        pathEscaper);

    this.modulesById = ImmutableMap.copyOf(modulesById);
  }

  @Override
  public @Nullable String resolveJsModule(
      String scriptAddress, String moduleAddress, String sourcename, int lineno, int colno) {
    String loadAddress = modulesById.get(moduleAddress);
    if (loadAddress == null) {
      // Module paths may still be used in type nodes so we need to fall back
      // to node module resolution for those.
      return super.resolveJsModule(scriptAddress, moduleAddress, sourcename, lineno, colno);
    }
    return loadAddress;
  }
}