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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 static com.google.common.base.Preconditions.checkNotNull;

import com.google.errorprone.annotations.CanIgnoreReturnValue;
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 java.util.ArrayDeque;
import java.util.Deque;
import org.jspecify.nullness.Nullable;

/** Converts ES6 arrow functions to standard anonymous ES3 functions. */
public class Es6RewriteArrowFunction implements NodeTraversal.Callback, CompilerPass {
  // The name of the vars that capture 'this' and 'arguments' for converting arrow functions. Note
  // that these names can be reused (once per scope) because declarations in nested scopes will
  // shadow one another, which results in the intended behaviour.
  private static final String ARGUMENTS_VAR = "$jscomp$arguments";
  static final String THIS_VAR = "$jscomp$this";

  private final AbstractCompiler compiler;
  private final AstFactory astFactory;
  private final Deque contextStack;
  private static final FeatureSet transpiledFeatures =
      FeatureSet.BARE_MINIMUM.with(Feature.ARROW_FUNCTIONS);

  public Es6RewriteArrowFunction(AbstractCompiler compiler) {
    this.compiler = compiler;
    this.astFactory = compiler.createAstFactory();
    this.contextStack = new ArrayDeque<>();
  }

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

  @Override
  public boolean shouldTraverse(NodeTraversal t, Node n, Node parent) {
    switch (n.getToken()) {
      case SCRIPT:
        contextStack.push(contextForScript(n));
        break;
      case FUNCTION:
        if (!n.isArrowFunction()) {
          contextStack.push(contextForFunction(n));
        }
        break;
      case SUPER:
        ThisAndArgumentsContext context = checkNotNull(contextStack.peek());
        // super(...) within a constructor.
        if (context.isConstructor && parent.isCall() && parent.getFirstChild() == n) {
          context.lastSuperStatement = getEnclosingStatement(parent, context.scopeBody);
        }
        break;
      default:
        break;
    }
    return true;
  }

  /**
   * @return The statement Node that is a child of block and contains n.
   */
  private Node getEnclosingStatement(Node n, Node block) {
    while (checkNotNull(n.getParent()) != block) {
      n = checkNotNull(NodeUtil.getEnclosingStatement(n.getParent()));
    }
    return n;
  }

  @Override
  public void visit(NodeTraversal t, Node n, Node parent) {
    ThisAndArgumentsContext context = contextStack.peek();

    if (n.isArrowFunction()) {
      visitArrowFunction(t, n, checkNotNull(context));
    } else if (context != null && context.scopeBody == n) {
      contextStack.pop();
      addVarDeclarations(t, context);
    }
  }

  private void visitArrowFunction(NodeTraversal t, Node n, ThisAndArgumentsContext context) {
    n.setIsArrowFunction(false);

    Node body = n.getLastChild();
    if (!body.isBlock()) {
      body.detach();
      body = IR.block(IR.returnNode(body)).srcrefTreeIfMissing(body);
      n.addChildToBack(body);
    }

    ThisAndArgumentsReferenceUpdater updater =
        new ThisAndArgumentsReferenceUpdater(compiler, context, astFactory);
    NodeTraversal.traverse(compiler, body, updater);

    t.reportCodeChange();
  }

  private void addVarDeclarations(NodeTraversal t, ThisAndArgumentsContext context) {
    Node scopeBody = context.scopeBody;

    if (context.needsThisVar) {
      Node name = astFactory.createName(THIS_VAR, context.getThisType());
      Node thisVar = IR.constNode(name, astFactory.createThis(context.getThisType()));
      NodeUtil.addFeatureToScript(t.getCurrentScript(), Feature.CONST_DECLARATIONS, compiler);
      thisVar.srcrefTreeIfMissing(scopeBody);
      makeTreeNonIndexable(thisVar);

      if (context.lastSuperStatement == null) {
        scopeBody.addChildToFront(thisVar);
      } else {
        // Not safe to reference `this` until after super() has been called.
        // TODO(bradfordcsmith): Some complex cases still aren't covered, like
        //     if (...) { super(); arrow function } else { super(); }
        thisVar.insertAfter(context.lastSuperStatement);
      }
      compiler.reportChangeToEnclosingScope(thisVar);
    }

    if (context.needsArgumentsVar) {
      Node argumentsVar = astFactory.createArgumentsAliasDeclaration(ARGUMENTS_VAR);
      NodeUtil.addFeatureToScript(t.getCurrentScript(), Feature.CONST_DECLARATIONS, compiler);
      scopeBody.addChildToFront(argumentsVar);

      argumentsVar.srcrefTreeIfMissing(scopeBody);
      compiler.reportChangeToEnclosingScope(argumentsVar);
    }
  }

  private void makeTreeNonIndexable(Node n) {
    n.makeNonIndexable();
    for (Node child = n.getFirstChild(); child != null; child = child.getNext()) {
      makeTreeNonIndexable(child);
    }
  }

  /**
   * Accumulates information about a scope in which `this` and `arguments` are consistent.
   *
   * 

Instances are maintained in a DFS stack during traversal of {@link Es6RewriteArrowFunction}. * They can't be immutable because a context isn't fully defined by a single node (`super()` makes * this hard). */ private static class ThisAndArgumentsContext { final Node scopeBody; final boolean isConstructor; @Nullable Node lastSuperStatement = null; // Last statement in the body that refers to super(). boolean needsThisVar = false; private AstFactory.@Nullable Type thisType; boolean needsArgumentsVar = false; private ThisAndArgumentsContext(Node scopeBody, boolean isConstructor) { this.scopeBody = scopeBody; this.isConstructor = isConstructor; } AstFactory.@Nullable Type getThisType() { return thisType; } @CanIgnoreReturnValue ThisAndArgumentsContext setNeedsThisVarWithType(AstFactory.Type type) { thisType = type; needsThisVar = true; return this; } @CanIgnoreReturnValue ThisAndArgumentsContext setNeedsArgumentsVar() { needsArgumentsVar = true; return this; } } private ThisAndArgumentsContext contextForFunction(Node functionNode) { Node scopeBody = functionNode.getLastChild(); return new ThisAndArgumentsContext(scopeBody, NodeUtil.isEs6Constructor(functionNode)); } private ThisAndArgumentsContext contextForScript(Node scriptNode) { return new ThisAndArgumentsContext(scriptNode, /* isConstructor= */ false); } /** * Sub-rewriter for references to `this` and `arguments` in a single arrow function. * *

An instance is generated for each arrow function in order of decreasing depth. This * isn't too inefficient, because instances don't traverse into non-arrow functions and all nested * functions will already have been "de-arrowed". */ private static class ThisAndArgumentsReferenceUpdater implements NodeTraversal.Callback { private final AbstractCompiler compiler; private final ThisAndArgumentsContext context; private final AstFactory astFactory; public ThisAndArgumentsReferenceUpdater( AbstractCompiler compiler, ThisAndArgumentsContext context, AstFactory astFactory) { this.compiler = compiler; this.context = context; this.astFactory = astFactory; } @Override public void visit(NodeTraversal t, Node n, Node parent) { if (n.isThis()) { context.setNeedsThisVarWithType(AstFactory.type(n)); Node name = astFactory.createName(THIS_VAR, context.getThisType()).srcref(n); name.makeNonIndexable(); if (compiler.getOptions().preservesDetailedSourceInfo()) { name.setOriginalName("this"); } n.replaceWith(name); } else if (n.isName() && n.getString().equals("arguments")) { context.setNeedsArgumentsVar(); Node name = astFactory.createName(ARGUMENTS_VAR, AstFactory.type(n)).srcref(n); if (compiler.getOptions().preservesDetailedSourceInfo()) { name.setOriginalName("arguments"); } n.replaceWith(name); } } @Override public boolean shouldTraverse(NodeTraversal t, Node n, Node parent) { return !n.isFunction() // TODO(nickreid): Remove this check? All functions below root should have already been // "de-arrowed". || n.isArrowFunction(); } } }





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