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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 2015 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.rhino.IR;
import com.google.javascript.rhino.JSDocInfoBuilder;
import com.google.javascript.rhino.JSTypeExpression;
import com.google.javascript.rhino.Node;
import com.google.javascript.rhino.Token;

/**
 * Converts ES6 arrow functions to standard anonymous ES3 functions.
 */
public class Es6RewriteArrowFunction extends NodeTraversal.AbstractPreOrderCallback
    implements HotSwapCompilerPass {
  private final AbstractCompiler compiler;

  // The name of the vars that capture 'this' and 'arguments'
  // for converting arrow functions.
  private static final String ARGUMENTS_VAR = "$jscomp$arguments";
  private static final String THIS_VAR = "$jscomp$this";

  public Es6RewriteArrowFunction(AbstractCompiler compiler) {
    this.compiler = compiler;
  }

  @Override
  public void process(Node externs, Node root) {
    NodeTraversal.traverseEs6(compiler, externs, this);
    NodeTraversal.traverseEs6(compiler, root, this);
  }

  @Override
  public void hotSwapScript(Node scriptRoot, Node originalRoot) {
    NodeTraversal.traverseEs6(compiler, scriptRoot, this);
  }

  @Override
  public boolean shouldTraverse(NodeTraversal t, Node n, Node parent) {
    switch (n.getType()) {
      case Token.FUNCTION:
        if (n.isArrowFunction()) {
          visitArrowFunction(t, n);
        }
        break;
    }
    return true;
  }

  private void visitArrowFunction(NodeTraversal t, Node n) {
    n.setIsArrowFunction(false);
    n.makeNonIndexable();
    Node body = n.getLastChild();
    if (!body.isBlock()) {
      body.detachFromParent();
      body = IR.block(IR.returnNode(body)).useSourceInfoIfMissingFromForTree(body);
      n.addChildToBack(body);
    }

    UpdateThisAndArgumentsReferences updater = new UpdateThisAndArgumentsReferences();
    NodeTraversal.traverseEs6(compiler, body, updater);
    addVarDecls(t, updater.changedThis, updater.changedArguments);

    compiler.reportCodeChange();
  }

  private void addVarDecls(NodeTraversal t, boolean addThis, boolean addArguments) {
    Scope scope = t.getScope().getClosestHoistScope();
    if (scope.isDeclared(THIS_VAR, false)) {
      addThis = false;
    }
    if (scope.isDeclared(ARGUMENTS_VAR, false)) {
      addArguments = false;
    }

    Node scopeRoot = scope.getRootNode();
    if (scopeRoot.isSyntheticBlock() && scopeRoot.getFirstChild().isScript()) {
      // Add the new node inside the SCRIPT node instead of the
      // synthetic block that contains it.
      scopeRoot = scopeRoot.getFirstChild();
    }

    CompilerInput input = compiler.getInput(scopeRoot.getInputId());
    if (addArguments) {
      Node name = IR.name(ARGUMENTS_VAR);
      Node argumentsVar = IR.constNode(name, IR.name("arguments"));
      JSDocInfoBuilder jsdoc = new JSDocInfoBuilder(false);
      jsdoc.recordType(
          new JSTypeExpression(
              new Node(Token.BANG, IR.string("Arguments")), ""));
      argumentsVar.setJSDocInfo(jsdoc.build());
      argumentsVar.useSourceInfoIfMissingFromForTree(scopeRoot);
      scopeRoot.addChildToFront(argumentsVar);
      scope.declare(ARGUMENTS_VAR, name, input);
    }
    if (addThis) {
      Node name = IR.name(THIS_VAR);
      Node thisVar = IR.constNode(name, IR.thisNode());
      thisVar.useSourceInfoIfMissingFromForTree(scopeRoot);
      scopeRoot.addChildToFront(thisVar);
      scope.declare(THIS_VAR, name, input);
    }
  }

  private static class UpdateThisAndArgumentsReferences implements NodeTraversal.Callback {
    private boolean changedThis = false;
    private boolean changedArguments = false;

    @Override
    public void visit(NodeTraversal t, Node n, Node parent) {
      if (n.isThis()) {
        Node name = IR.name(THIS_VAR).srcref(n);
        parent.replaceChild(n, name);
        changedThis = true;
      } else if (n.isName() && n.getString().equals("arguments")) {
        Node name = IR.name(ARGUMENTS_VAR).srcref(n);
        parent.replaceChild(n, name);
        changedArguments = true;
      }
    }

    @Override
    public boolean shouldTraverse(NodeTraversal t, Node n, Node parent) {
      return !n.isFunction() || n.isArrowFunction();
    }
  }
}