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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 2020 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 static com.google.common.base.Preconditions.checkArgument;

import com.google.javascript.rhino.Node;
import com.google.javascript.rhino.Token;

/** Utilities to abstract away certain common AST manipulations */
public final class AstManipulations {

  // Non-instantiable
  private AstManipulations() {}

  /**
   * Returns a single node equivalent to executing {@code }.
   *
   * Requires that expr1 and expr2 are detached (i.e. have no parent nodes). If exp2 is EMPTY
   * this just returns exp1.
   */
  public static Node fuseExpressions(Node exp1, Node exp2) {
    checkArgument(exp1.getParent() == null, "Expected detached node, got %s", exp1);
    checkArgument(exp2.getParent() == null, "Expected detached node, got %s", exp2);
    if (exp2.isEmpty()) {
      return exp1;
    }
    Node comma = new Node(Token.COMMA, exp1);
    comma.srcrefIfMissing(exp2);

    // We can just join the new comma expression with another comma but
    // lets keep all the comma's in a straight line. That way we can use
    // tree comparison.
    if (exp2.isComma()) {
      Node leftMostChild = exp2;
      while (leftMostChild.isComma()) {
        leftMostChild = leftMostChild.getFirstChild();
      }
      Node parent = leftMostChild.getParent();
      comma.addChildToBack(leftMostChild.detach());
      parent.addChildToFront(comma);
      return exp2;
    } else {
      comma.addChildToBack(exp2);
      return comma;
    }
  }
}