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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.lint;

import com.google.javascript.jscomp.AbstractCompiler;
import com.google.javascript.jscomp.CodePrinter;
import com.google.javascript.jscomp.CompilerPass;
import com.google.javascript.jscomp.DiagnosticType;
import com.google.javascript.jscomp.NodeTraversal;
import com.google.javascript.jscomp.NodeUtil;
import com.google.javascript.rhino.Node;

/**
 * Check that the goog.module does not use a default export.
 *
 * Example code:
 *
 * 
{@code class C {...} exports = C; }
 *
 * Must become:
 *
 * 
{@code class C {...} exports = {C}; }
 *
 * Which requires changes on the importer side from {@code const C = goog.require} to {@code
 * const {C} = goog.require}.
 */
public final class CheckDefaultExportOfGoogModule implements NodeTraversal.Callback, CompilerPass {

  private static final String WARNING_PREFIX =
      "Default exports of goog.modules "
          + "do not translate easily to ES module semantics.";

  public static final DiagnosticType DEFAULT_EXPORT_IN_GOOG_MODULE =
      DiagnosticType.disabled(
          "JSC_DEFAULT_EXPORT_IN_GOOG_MODULE",
          WARNING_PREFIX
              + " Please use named exports instead (`exports = '{'{0}'}';`) and change the import"
              + " sites to use destructuring (`const '{'{0}'}' = goog.require(''...'');`)."
          );

  public static final DiagnosticType MAYBE_ACCIDENTAL_DEFAULT_EXPORT_IN_GOOG_MODULE =
      DiagnosticType.disabled(
          "JSC_MAYBE_ACCIDENTAL_DEFAULT_EXPORT_IN_GOOG_MODULE",
          WARNING_PREFIX
              + " The exports pattern \n"
              + "{0} is a special case of default exports in JSCompiler as one of its keys is not"
              + " initialized with a local name, and therefore it can not be destructured at the"
              + " import site. Please use named exports instead."
          );

  private final AbstractCompiler compiler;

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

  @Override
  public boolean shouldTraverse(NodeTraversal t, Node n, Node parent) {
    if (n.isScript()) {
      return n.getBooleanProp(Node.GOOG_MODULE);
    }
    return true;
  }

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

  @Override
  public void visit(NodeTraversal t, Node n, Node parent) {
    if (isGoogModuleDefaultExportsAssignment(n)) {
      Node rhs = n.getFirstChild().getSecondChild();
      if (rhs.isName()) {
        // e.g `exports = Foo;`
        String exportedName = rhs.getString();
        t.report(n, DEFAULT_EXPORT_IN_GOOG_MODULE, exportedName);
      } else {
        if (rhs.isObjectLit()) {
          // accidental default export. e.g. `exports = {foo : 0}`
          int maxLength = 40;
          String codeSnippet = new CodePrinter.Builder(n).setPrettyPrint(true).build();
          if (codeSnippet.length() > maxLength) {
            // limit to prevent error message length from exploding for large object literals
            codeSnippet = codeSnippet.substring(0, maxLength) + "...};\n";
          }
          t.report(n, MAYBE_ACCIDENTAL_DEFAULT_EXPORT_IN_GOOG_MODULE, codeSnippet);
        } else {
          // e.g. `exports = class Foo {}`
          t.report(n, DEFAULT_EXPORT_IN_GOOG_MODULE, "MyVariable");
        }
      }
    }
  }

  /**
   * Matches default export assignments `exports = ..;` except when r.h.s is object literal like
   * `exports = {Foo};` since it treated as named exports.
   */
  private static boolean isGoogModuleDefaultExportsAssignment(Node statement) {
    if (!statement.isExprResult()) {
      return false;
    }

    if (!statement.getFirstChild().isAssign()) {
      return false;
    }

    if (!statement.getFirstFirstChild().isName()) {
      return false;
    }

    if (NodeUtil.isNamedExportsLiteral(statement.getFirstChild().getSecondChild())) {
      return false;
    }

    return statement.getFirstFirstChild().getString().equals("exports");
  }
}