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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 2018 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 com.google.javascript.jscomp.parsing.parser.FeatureSet;
import com.google.javascript.jscomp.parsing.parser.FeatureSet.Feature;
import com.google.javascript.rhino.Node;

/**
 * Converts object spread to valid ES2017 code.
 *
 * Currently this class converts Object spread properties as documented in tc39.
 * https://github.com/tc39/proposal-object-rest-spread. For example:
 *
 * 
{@code const bar = {a: 1, ...foo};}
 *
 * Note that object rest is handled by {@link Es6RewriteDestructuring}
 */
public final class RewriteObjectSpread implements NodeTraversal.Callback, HotSwapCompilerPass {
  private final AbstractCompiler compiler;
  private static final FeatureSet transpiledFeatures =
      FeatureSet.BARE_MINIMUM.with(Feature.OBJECT_LITERALS_WITH_SPREAD);

  private final AstFactory astFactory;

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

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

  @Override
  public void hotSwapScript(Node scriptRoot, Node originalRoot) {
    TranspilationPasses.hotSwapTranspile(compiler, scriptRoot, transpiledFeatures, this);
    TranspilationPasses.maybeMarkFeaturesAsTranspiledAway(compiler, transpiledFeatures);
  }

  @Override
  public boolean shouldTraverse(NodeTraversal t, Node n, Node parent) {
    return true;
  }

  @Override
  public void visit(NodeTraversal t, Node n, Node parent) {
    switch (n.getToken()) {
      case OBJECTLIT:
        visitObject(t, n);
        break;
      default:
        break;
    }
  }

  private void visitObject(NodeTraversal t, Node obj) {
    for (Node child : obj.children()) {
      if (child.isSpread()) {
        visitObjectWithSpread(t, obj);
        return;
      }
    }
  }

  /*
   * Convert '{first: b, c, ...spread, d: e, last}' to:
   *
   * Object.assign({}, {first:b, c}, spread, {d:e, last});
   */
  private void visitObjectWithSpread(NodeTraversal t, Node obj) {
    checkArgument(obj.isObjectLit());

    // Add an empty target object literal so changes made by Object.assign will not affect any other
    // variables.
    Node result =
        astFactory.createObjectDotAssignCall(
            t.getScope(), obj.getJSType(), astFactory.createObjectLit());

    // An indicator whether the current last thing in the param list is an object literal to which
    // properties may be added.  Initialized to null since nothing should be added to the empty
    // object literal in first position of the param list.
    Node trailingObjectLiteral = null;

    for (Node child : obj.children()) {
      if (child.isSpread()) {
        // Add the object directly to the param list.
        Node spreaded = child.removeFirstChild();
        result.addChildToBack(spreaded);

        // Properties should not be added to the trailing object.
        trailingObjectLiteral = null;
      } else {
        if (trailingObjectLiteral == null) {
          // Add a new object to which properties may be added.
          trailingObjectLiteral = astFactory.createObjectLit();
          result.addChildToBack(trailingObjectLiteral);
        }
        // Add the property to the object literal.
        trailingObjectLiteral.addChildToBack(child.detach());
      }
    }

    result.useSourceInfoIfMissingFromForTree(obj);
    obj.replaceWith(result);
    compiler.reportChangeToEnclosingScope(result);
  }
}