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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.jscomp;

import com.google.javascript.jscomp.NodeTraversal.AbstractPostOrderCallback;
import com.google.javascript.jscomp.parsing.parser.FeatureSet;
import com.google.javascript.jscomp.parsing.parser.FeatureSet.Feature;
import com.google.javascript.rhino.IR;
import com.google.javascript.rhino.Node;
import com.google.javascript.rhino.Token;

/**
 * Rewrite block-scoped function declarations as "let"s. This pass must happen before
 * Es6RewriteBlockScopedDeclaration, which rewrites "let" to "var".
 */
public final class Es6RewriteBlockScopedFunctionDeclaration extends AbstractPostOrderCallback
    implements CompilerPass {

  private final AbstractCompiler compiler;
  private static final FeatureSet transpiledFeatures =
      FeatureSet.BARE_MINIMUM.with(Feature.BLOCK_SCOPED_FUNCTION_DECLARATION);

  public Es6RewriteBlockScopedFunctionDeclaration(AbstractCompiler compiler) {
    this.compiler = compiler;
  }

  @Override
  public void process(Node externs, Node root) {
    TranspilationPasses.processTranspile(compiler, root, transpiledFeatures, this);
    TranspilationPasses.maybeMarkFeaturesAsTranspiledAway(compiler, transpiledFeatures);
  }

  @Override
  public void visit(NodeTraversal t, Node n, Node parent) {
    if (n.isFunction()
        && parent != null
        && parent.isBlock()
        && !parent.getParent().isFunction()) {
      // Only consider declarations (all expressions have non-block parents) that are not directly
      // within a function or top-level.
      visitBlockScopedFunctionDeclaration(t, n, parent);
    }
  }

  /**
   * Rewrite the function declaration from:
   *
   *    *   function f() {}
   *   FUNCTION
   *     NAME x
   *     PARAM_LIST
   *     BLOCK
   * 
   *
   * to
   *
   *    *   let f = function() {};
   *   LET
   *     NAME f
   *       FUNCTION
   *         NAME (w/ empty string)
   *         PARAM_LIST
   *         BLOCK
   * 
   *
   * This is similar to {@link Normalize.NormalizeStatements#rewriteFunctionDeclaration} but
   * rewrites to "let" instead of "var".
   */
  private void visitBlockScopedFunctionDeclaration(NodeTraversal t, Node n, Node parent) {
    // Prepare a spot for the function.
    Node oldNameNode = n.getFirstChild();
    Node fnNameNode = oldNameNode.cloneNode();
    Node let = IR.declaration(fnNameNode, Token.LET).srcref(n);
    NodeUtil.addFeatureToScript(t.getCurrentScript(), Feature.LET_DECLARATIONS, compiler);

    // Prepare the function.
    oldNameNode.setString("");
    compiler.reportChangeToEnclosingScope(oldNameNode);

    // Move the function to the front of the parent.
    n.detach();
    parent.addChildToFront(let);
    compiler.reportChangeToEnclosingScope(let);
    fnNameNode.addChildToFront(n);
  }
}