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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.refactoring.examples;

import com.google.common.collect.ImmutableList;
import com.google.javascript.jscomp.AbstractCompiler;
import com.google.javascript.refactoring.Match;
import com.google.javascript.refactoring.NodeMetadata;
import com.google.javascript.refactoring.Scanner;
import com.google.javascript.refactoring.SuggestedFix;
import com.google.javascript.rhino.Node;
import com.google.javascript.rhino.QualifiedName;

/**
 * Replaces goog.bind(..., this) with arrow functions. The pretty-printer does not do well with
 * arrow functions, so it is recommended that you run 'g4 fix' to invoke clang-format on the CLs
 * created by this refactoring.
 *
 * TODO(tbreisacher): Handle (function(){}).bind(this); as well.
 */
public final class GoogBindToArrow extends Scanner {
  private static final QualifiedName GOOG_BIND_NAME = QualifiedName.of("goog.bind");

  private static boolean isGoogBind(Node n) {
    return n.isGetProp() && GOOG_BIND_NAME.matches(n);
  }

  @Override
  public boolean matches(Node node, NodeMetadata metadata) {
    if (!node.isCall()) {
      return false;
    }
    Node callee = node.getFirstChild();
    if (!isGoogBind(callee)) {
      return false;
    }

    Node firstArg = callee.getNext();
    if (firstArg == null || !firstArg.isFunction()) {
      return false;
    }

    Node secondArg = firstArg.getNext();
    return secondArg != null && secondArg.isThis() && secondArg.getNext() == null;
  }

  @Override
  public ImmutableList processMatch(Match match) {
    AbstractCompiler compiler = match.getMetadata().getCompiler();
    Node googBindCall = match.getNode();
    Node function = googBindCall.getFirstChild().getNext();
    Node arrowFunction = function.cloneTree();
    arrowFunction.setIsArrowFunction(true);

    // For "function() { return x; }", transform to "() => x" instead of "() => { return x; }".
    Node body = function.getLastChild();
    if (body.hasOneChild()) {
      Node returnNode = body.getFirstChild();
      if (returnNode.isReturn()) {
        arrowFunction.getLastChild().replaceWith(returnNode.removeFirstChild());
      }
    }

    SuggestedFix.Builder fix = new SuggestedFix.Builder();
    fix.replace(googBindCall, arrowFunction, compiler);

    return ImmutableList.of(fix.build());
  }
}