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• CollapseAnonymousFunctions.java

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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;
  }
}