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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 2021 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 static com.google.common.base.Preconditions.checkState;
import static com.google.javascript.jscomp.AstFactory.type;

import com.google.errorprone.annotations.CheckReturnValue;
import com.google.javascript.jscomp.parsing.parser.FeatureSet.Feature;
import com.google.javascript.rhino.IR;
import com.google.javascript.rhino.JSDocInfo;
import com.google.javascript.rhino.Node;

/** Adds explicit constructors to classes that lack them. */
@CheckReturnValue
final class SynthesizeExplicitConstructors {
  private final AbstractCompiler compiler;
  private final AstFactory astFactory;

  SynthesizeExplicitConstructors(AbstractCompiler compiler) {
    this.compiler = compiler;
    this.astFactory = compiler.createAstFactory();
  }

  /** Add a synthetic constructor to the given class if no constructor is present */
  void synthesizeClassConstructorIfMissing(NodeTraversal t, Node classNode) {
    checkArgument(classNode.isClass());
    if (NodeUtil.getEs6ClassConstructorMemberFunctionDef(classNode) == null) {
      addSyntheticConstructor(t, classNode);
    }
  }

  private void addSyntheticConstructor(NodeTraversal t, Node classNode) {
    Node superClass = classNode.getSecondChild();
    Node classMembers = classNode.getLastChild();
    Node memberDef;
    if (superClass.isEmpty()) {
      Node function = astFactory.createEmptyFunction(type(classNode));
      compiler.reportChangeToChangeScope(function);
      memberDef = astFactory.createMemberFunctionDef("constructor", function);
    } else {
      checkState(
          superClass.isQualifiedName(),
          "Expected Es6RewriteClassExtendsExpressions to make all extends clauses into qualified"
              + " names, found %s",
          superClass);
      Node body = IR.block();

      // If a class is defined in an externs file or as an interface, it's only a stub, not an
      // implementation that should be instantiated.
      // A call to super() shouldn't actually exist for these cases and is problematic to
      // transpile, so don't generate it.
      if (!classNode.isFromExterns() && !isInterface(classNode)) {
        // Generate required call to super()
        // `super(...arguments);`
        // Note that transpilation of spread args must occur after this pass for this to work.
        Node exprResult =
            IR.exprResult(
                astFactory.createConstructorCall(
                    type(classNode), // returned type is the subclass
                    astFactory.createSuper(type(superClass)),
                    IR.iterSpread(astFactory.createArgumentsReference())));
        body.addChildToFront(exprResult);
        NodeUtil.addFeatureToScript(t.getCurrentScript(), Feature.SUPER, compiler);
        NodeUtil.addFeatureToScript(t.getCurrentScript(), Feature.SPREAD_EXPRESSIONS, compiler);
      }
      Node constructor = astFactory.createFunction("", IR.paramList(), body, type(classNode));
      memberDef = astFactory.createMemberFunctionDef("constructor", constructor);
    }
    memberDef.srcrefTreeIfMissing(classNode);
    memberDef.makeNonIndexableRecursive();
    classMembers.addChildToFront(memberDef);
    NodeUtil.addFeatureToScript(t.getCurrentScript(), Feature.MEMBER_DECLARATIONS, compiler);
    // report newly created constructor
    compiler.reportChangeToChangeScope(memberDef.getOnlyChild());
    // report change to scope containing the class
    compiler.reportChangeToEnclosingScope(memberDef);
  }

  private boolean isInterface(Node classNode) {
    JSDocInfo classJsDocInfo = NodeUtil.getBestJSDocInfo(classNode);
    return classJsDocInfo != null && classJsDocInfo.isInterface();
  }
}