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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 2020 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.AstFactory.type;

import com.google.common.base.Supplier;
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;

/** Replaces the ES2020 `??` operator with conditional (?:). */
public final class RewriteNullishCoalesceOperator implements NodeTraversal.Callback, CompilerPass {

  private static final String TEMP_VAR_NAME_PREFIX = "$jscomp$nullish$tmp";

  private final AbstractCompiler compiler;
  private final AstFactory astFactory;
  private final Supplier uniqueNameIdSuppier;

  public RewriteNullishCoalesceOperator(AbstractCompiler compiler) {
    this.compiler = compiler;
    this.astFactory = compiler.createAstFactory();
    this.uniqueNameIdSuppier = compiler.getUniqueNameIdSupplier();
  }

  @Override
  public void process(Node externs, Node root) {
    NodeTraversal.traverseRoots(compiler, new ArrowRewriteCallBack(), externs, root);
    NodeTraversal.traverse(compiler, root, this);
    TranspilationPasses.maybeMarkFeaturesAsTranspiledAway(compiler, Feature.NULL_COALESCE_OP);
  }

  @Override
  public boolean shouldTraverse(NodeTraversal t, Node n, Node parent) {
    if (n.isScript()) {
      FeatureSet scriptFeatures = NodeUtil.getFeatureSetOfScript(n);
      return scriptFeatures == null || scriptFeatures.contains(Feature.NULL_COALESCE_OP);
    }
    return true;
  }

  @Override
  public void visit(NodeTraversal t, Node n, Node parent) {
    if (n.isNullishCoalesce()) {
      visitNullishCoalesce(t, n);
      t.reportCodeChange();
    }
  }

  private void visitNullishCoalesce(NodeTraversal t, Node n) {
    // a() ?? b()
    // let temp;
    // ((temp = a()) != null) ? temp : b()
    String tempVarName = TEMP_VAR_NAME_PREFIX + uniqueNameIdSuppier.get();
    Node enclosingStatement = NodeUtil.getEnclosingStatement(n);

    Node left = n.removeFirstChild();
    Node right = n.getLastChild().detach();

    Node let = astFactory.createSingleLetNameDeclaration(tempVarName);
    Node assignName = astFactory.createName(tempVarName, type(left));
    Node assign = astFactory.createAssign(assignName, left);
    Node ne = astFactory.createNe(assign, astFactory.createNull());
    Node hookName = astFactory.createName(tempVarName, type(left));
    Node hook = astFactory.createHook(ne, hookName, right);

    let.srcrefTreeIfMissing(left);
    assignName.srcrefTreeIfMissing(left);
    assign.srcrefTreeIfMissing(left);
    ne.srcrefTreeIfMissing(left);
    hookName.srcrefTreeIfMissing(left);

    let.insertBefore(enclosingStatement);
    n.replaceWith(hook);

    NodeUtil.addFeatureToScript(t.getCurrentScript(), Feature.LET_DECLARATIONS, compiler);
    compiler.reportChangeToEnclosingScope(hook);
  }

  /**
   * Nullish coalesce rewriting declares new variables within the enclosing scope of the nullish
   * coalesce operator. This callback makes arrow functions have a block around the body to prevent
   * new declarations from leaking into the higher scope.
   */
  //  TODO(b/197349249) Rewriting arrows would become unnecessary if this pass ran normalized.
  private class ArrowRewriteCallBack implements NodeTraversal.Callback {

    @Override
    public boolean shouldTraverse(NodeTraversal t, Node n, Node parent) {
      if (n.isScript()) {
        FeatureSet scriptFeatures = NodeUtil.getFeatureSetOfScript(n);
        return scriptFeatures == null || scriptFeatures.contains(Feature.NULL_COALESCE_OP);
      }
      return true;
    }

    @Override
    public void visit(NodeTraversal t, Node n, Node parent) {
      if (!n.isNullishCoalesce()) {
        return;
      }
      Node arrowOrBlock =
          NodeUtil.getEnclosingNode(n, (node) -> node.isArrowFunction() || node.isBlock());
      if (arrowOrBlock != null && arrowOrBlock.isArrowFunction()) {
        if (!NodeUtil.getFunctionBody(arrowOrBlock).isBlock()) {
          // create a block around the arrow function body.
          Node returnValue = NodeUtil.getFunctionBody(arrowOrBlock);
          Node body = IR.block(IR.returnNode(returnValue.detach()));
          body.srcrefTreeIfMissing(returnValue);
          arrowOrBlock.addChildToBack(body);
          compiler.reportChangeToEnclosingScope(body);
        }
      }
    }
  }
}