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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 2008 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.jscomp.NodeTraversal.AbstractPostOrderCallback;
import com.google.javascript.rhino.Node;

/**
 * Collapses anonymous function expressions into named function declarations,
 * i.e. the following:
 *
 *  * var f = function() {}
 *  *
 * becomes:
 *
 * function f() {}
 *
 * This reduces the generated code size but changes the semantics because f
 * will be defined before its definition is reached.
 * Also, in ES6+, "var f" is visible in the entire function scope, whereas
 * "function f" is block scoped, which may cause issues.
 */
class CollapseAnonymousFunctions extends AbstractPostOrderCallback implements CompilerPass {
  private final AbstractCompiler compiler;

  public CollapseAnonymousFunctions(AbstractCompiler compiler) {
    checkArgument(compiler.getLifeCycleStage().isNormalized());
    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) {
    if (!(n.isVar() || n.isLet() || n.isConst())) {
      return;
    }

    // It is only safe to collapse anonymous functions that appear
    // at top-level blocks.  In other cases the difference between
    // variable and function declarations can lead to problems or
    // expose subtle bugs in browser implementation as function
    // definitions are added to scopes before the start of execution.

    Node grandparent = parent.getParent();
    if (!(parent.isScript()
        || (grandparent != null && grandparent.isFunction() && parent.isBlock()))) {
      return;
    }

    // Need to store the next name in case the current name is removed from
    // the linked list.
    Node name = n.getOnlyChild();

    // Don't collapse if the lhs is a destructuring pattern.
    if (!name.isName()) {
      return;
    }

    Node value = name.getFirstChild();
    if (value != null
        && value.isFunction()
        && !value.isArrowFunction()
        && !isRecursiveFunction(value)) {
      Node fnName = value.getFirstChild();
      fnName.setString(name.getString());
      NodeUtil.copyNameAnnotations(name, fnName);
      value.detach();
      n.replaceWith(value);

      // Renormalize the code.
      if (!t.inGlobalScope() && NodeUtil.isHoistedFunctionDeclaration(value)) {
        parent.addChildToFront(value.detach());
      }

      // report changes to both the change scopes
      compiler.reportChangeToChangeScope(value);
      t.reportCodeChange();
    }
  }

  private boolean isRecursiveFunction(Node function) {
    Node name = function.getFirstChild();
    if (name.getString().isEmpty()) {
      return false;
    }
    Node args = name.getNext();
    Node body = args.getNext();
    return containsName(body, name.getString());
  }

  private boolean containsName(Node n, String name) {
    if (n.isName() && n.getString().equals(name)) {
      return true;
    }

    for (Node child = n.getFirstChild(); child != null; child = child.getNext()) {
      if (containsName(child, name)) {
        return true;
      }
    }
    return false;
  }
}