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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 2014 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.javascript.jscomp.Es6ToEs3Converter.CANNOT_CONVERT_YET;

import com.google.common.base.Joiner;
import com.google.common.base.Preconditions;
import com.google.javascript.rhino.IR;
import com.google.javascript.rhino.Node;

/**
 * Converts {@code super} nodes. This has to run before the main
 * {@link Es6ToEs3Converter} pass.
 */
public final class Es6ConvertSuper implements NodeTraversal.Callback, HotSwapCompilerPass {
  private final AbstractCompiler compiler;

  public Es6ConvertSuper(AbstractCompiler compiler) {
    this.compiler = compiler;
  }

  @Override
  public boolean shouldTraverse(NodeTraversal t, Node n, Node parent) {
    if (n.isClass()) {
      boolean hasConstructor = false;
      for (Node member = n.getLastChild().getFirstChild();
          member != null;
          member = member.getNext()) {
        if (member.isMemberFunctionDef() && member.getString().equals("constructor")) {
          hasConstructor = true;
        }
      }
      if (!hasConstructor) {
        addSyntheticConstructor(n);
      }
    }
    return true;
  }

  @Override
  public void visit(NodeTraversal t, Node n, Node parent) {
    if (n.isSuper()) {
      visitSuper(n, parent);
    }
  }

  private void addSyntheticConstructor(Node classNode) {
    Node superClass = classNode.getSecondChild();
    Node classMembers = classNode.getLastChild();
    Node memberDef;
    if (superClass.isEmpty()) {
      memberDef = IR.memberFunctionDef("constructor",
          IR.function(IR.name(""), IR.paramList(), IR.block()));
    } else {
      if (!superClass.isQualifiedName()) {
        // This will be reported as an error in Es6ToEs3Converter.
        return;
      }
      Node body = IR.block();
      if (!classNode.isFromExterns()) {
        Node exprResult = IR.exprResult(IR.call(
            IR.getprop(superClass.cloneTree(), IR.string("apply")),
            IR.thisNode(),
            IR.name("arguments")));
        body.addChildrenToFront(exprResult);
      }
      Node constructor = IR.function(
          IR.name(""),
          IR.paramList(IR.name("var_args")),
          body);
      memberDef = IR.memberFunctionDef("constructor", constructor);
    }
    memberDef.useSourceInfoIfMissingFromForTree(classNode);
    classMembers.addChildToFront(memberDef);
  }

  private void visitSuper(Node node, Node parent) {
    Node enclosingCall = parent;
    Node potentialCallee = node;
    if (!parent.isCall()) {
      enclosingCall = parent.getParent();
      potentialCallee = parent;
    }
    if (!enclosingCall.isCall()
        || enclosingCall.getFirstChild() != potentialCallee
        || enclosingCall.getFirstChild().isGetElem()) {
      compiler.report(JSError.make(node, CANNOT_CONVERT_YET,
          "Only calls to super or to a method of super are supported."));
      return;
    }
    Node clazz = NodeUtil.getEnclosingClass(node);
    if (NodeUtil.getNameNode(clazz) == null) {
      // Unnamed classes of the form:
      //   f(class extends D { ... });
      // will be rejected when the class is processed.
      return;
    }

    Node superName = clazz.getSecondChild();
    if (!superName.isQualifiedName()) {
      // This will be reported as an error in Es6ToEs3Converter.
      return;
    }

    Node enclosingMemberDef = NodeUtil.getEnclosingClassMemberFunction(node);
    if (enclosingMemberDef.isStaticMember()) {
      Node callTarget;
      potentialCallee.detachFromParent();
      if (potentialCallee == node) {
        // of the form super()
        potentialCallee =
            IR.getprop(superName.cloneTree(), IR.string(enclosingMemberDef.getString()));
        enclosingCall.putBooleanProp(Node.FREE_CALL, false);
      } else {
        // of the form super.method()
        potentialCallee.replaceChild(node, superName.cloneTree());
      }
      callTarget = IR.getprop(potentialCallee, IR.string("call"));
      enclosingCall.addChildToFront(callTarget);
      enclosingCall.addChildAfter(IR.thisNode(), callTarget);
      enclosingCall.useSourceInfoIfMissingFromForTree(enclosingCall);
      compiler.reportCodeChange();
      return;
    }

    String methodName;
    Node callName = enclosingCall.removeFirstChild();
    if (callName.isSuper()) {
      methodName = enclosingMemberDef.getString();
    } else {
      methodName = callName.getLastChild().getString();
    }
    Node baseCall = baseCall(
        superName.getQualifiedName(), methodName, enclosingCall.removeChildren());
    baseCall.useSourceInfoIfMissingFromForTree(enclosingCall);
    enclosingCall.getParent().replaceChild(enclosingCall, baseCall);
    compiler.reportCodeChange();
  }

  private Node baseCall(String baseClass, String methodName, Node arguments) {
    Preconditions.checkNotNull(baseClass);
    Preconditions.checkNotNull(methodName);
    String baseMethodName;
    if (methodName.equals("constructor")) {
      baseMethodName = baseClass + ".call";
    } else {
      baseMethodName = Joiner.on('.').join(baseClass, "prototype", methodName, "call");
    }
    Node methodCall = NodeUtil.newQName(compiler, baseMethodName);
    Node callNode = IR.call(methodCall, IR.thisNode());
    if (arguments != null) {
      callNode.addChildrenToBack(arguments);
    }
    return callNode;
  }

  @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);
  }
}