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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;

import com.google.javascript.jscomp.NodeTraversal.AbstractPostOrderCallback;
import com.google.javascript.rhino.Node;

/**
 * Checks that ES6 Modules are used correctly, and do not reference undefined keywords or features.
 */
public final class Es6CheckModule extends AbstractPostOrderCallback implements CompilerPass {
  static final DiagnosticType ES6_MODULE_REFERENCES_THIS =
      DiagnosticType.warning(
          "ES6_MODULE_REFERENCES_THIS", "The body of an ES6 module cannot reference 'this'.");

  static final DiagnosticType IMPORT_CANNOT_BE_REASSIGNED =
      DiagnosticType.error(
          "JSC_IMPORT_CANNOT_BE_REASSIGNED", "Assignment to constant variable \"{0}\".");

  private final AbstractCompiler compiler;

  public Es6CheckModule(AbstractCompiler compiler) {
    this.compiler = compiler;
  }

  @Override
  public void process(Node externs, Node root) {
    NodeTraversal.traverse(compiler, root, this);
  }

  @Override
  public void visit(NodeTraversal t, Node n, Node parent) {
    switch (n.getToken()) {
      case THIS:
        if (t.inModuleHoistScope()) {
          t.report(n, ES6_MODULE_REFERENCES_THIS);
        }
        break;
      case GETPROP:
      case GETELEM:
        if (NodeUtil.isLValue(n)
            && !NodeUtil.isDeclarationLValue(n)
            && n.getFirstChild().isName()) {
          Var var = t.getScope().getVar(n.getFirstChild().getString());
          if (var != null) {
            Node nameNode = var.getNameNode();
            if (nameNode != null && nameNode.isImportStar()) {
              // import * as M from '';
              // M.x = 2;
              compiler.report(JSError.make(n, IMPORT_CANNOT_BE_REASSIGNED, nameNode.getString()));
            }
          }
        }
        break;
      case NAME:
        if (NodeUtil.isLValue(n) && !NodeUtil.isDeclarationLValue(n)) {
          Var var = t.getScope().getVar(n.getString());
          if (var != null) {
            Node nameNode = var.getNameNode();
            if (nameNode != null && nameNode != n) {
              if (NodeUtil.isImportedName(nameNode)) {
                // import { x } from '';
                // x = 2;
                compiler.report(JSError.make(n, IMPORT_CANNOT_BE_REASSIGNED, nameNode.getString()));
              }
            }
          }
        }
        break;
      default:
        break;
    }
  }
}