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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;

import com.google.common.annotations.GwtIncompatible;
import com.google.javascript.jscomp.NodeTraversal.AbstractPreOrderCallback;
import com.google.javascript.jscomp.deps.ModuleLoader;
import com.google.javascript.rhino.Node;
import java.net.URI;
import java.net.URISyntaxException;
import java.nio.file.Path;

/**
 * Rewrites ES6 import paths to be relative after resolving according to the compiler's module
 * resolver.
 *
 * Useful for servers that wish to preserve ES6 modules, meaning their paths need to be valid in
 * the browser.
 */
@GwtIncompatible("java.net.URI")
public class Es6RelativizeImportPaths implements CompilerPass {

  private final AbstractCompiler compiler;

  public Es6RelativizeImportPaths(AbstractCompiler compiler) {
    this.compiler = compiler;
  }

  @Override
  public void process(Node externs, Node root) {
    for (Node script = root.getFirstChild(); script != null; script = script.getNext()) {
      if (Es6RewriteModules.isEs6ModuleRoot(script)) {
        NodeTraversal.traverse(compiler, script, new Rewriter());
      }
    }
  }

  private static class Rewriter extends AbstractPreOrderCallback {
    @Override
    public boolean shouldTraverse(NodeTraversal nodeTraversal, Node n, Node parent) {
      switch (n.getToken()) {
        case ROOT:
        case MODULE_BODY:
        case SCRIPT:
          return true;
        case IMPORT:
          visitImport(nodeTraversal, n);
          return false;
        case EXPORT:
          visitExport(nodeTraversal, n);
          return false;
        default:
          return false;
      }
    }

    private void visitImport(NodeTraversal t, Node importDecl) {
      Node specifierNode = importDecl.getLastChild();
      String specifier = specifierNode.getString();

      // Leave relative and truly absolute paths (those with a scheme) alone. Only transform
      // absolute paths without a scheme (starting with "/") and ambiguous paths.
      if (ModuleLoader.isRelativeIdentifier(specifier)) {
        return;
      } else {
        try {
          URI specifierURI = new URI(specifier);
          if (specifierURI.isAbsolute()) {
            return;
          }
        } catch (URISyntaxException e) {
          return;
        }
      }

      String scriptPath = t.getInput().getPath().toString();

      try {
        // If the script path has a scheme / host / port then just use the path part.
        scriptPath = new URI(scriptPath).getPath();
      } catch (URISyntaxException e) {
        return;
      }

      String newSpecifier = t.getInput().getPath().resolveModuleAsPath(specifier).toString();

      // If a module root is stripped then this won't start with "/" when it probably should.
      if (!newSpecifier.startsWith("/")) {
        newSpecifier = "/" + newSpecifier;
      }

      newSpecifier = Path.of(scriptPath).getParent().relativize(Path.of(newSpecifier)).toString();

      // Relativizing two paths with the same directory yields an ambiguous path rather than one
      // starting with "./".
      if (ModuleLoader.isAmbiguousIdentifier(newSpecifier)) {
        newSpecifier = "./" + newSpecifier;
      }

      if (!newSpecifier.equals(specifier)) {
        specifierNode.setString(newSpecifier);
        t.reportCodeChange(specifierNode);
      }
    }

    private void visitExport(NodeTraversal t, Node export) {
      if (export.hasTwoChildren()) {
        // export from
        visitImport(t, export);
      }
    }
  }
}