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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.checkState;
import static com.google.javascript.jscomp.AstFactory.type;

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

/* Helper class for RewriteLogicalAssignmentOperators */
class RewriteLogicalAssignmentOperatorsHelper {

  private static final String TEMP_VAR_NAME_PREFIX = "$jscomp$logical$assign$tmp";
  private static final String TEMP_INDEX_VAR_NAME_PREFIX = "$jscomp$logical$assign$tmpindex";

  private final AbstractCompiler compiler;
  private final AstFactory astFactory;
  private final UniqueIdSupplier uniqueIdSupplier;

  public RewriteLogicalAssignmentOperatorsHelper(
      AbstractCompiler compiler, AstFactory astFactory, UniqueIdSupplier uniqueIdSupplier) {
    this.compiler = compiler;
    this.astFactory = astFactory;
    this.uniqueIdSupplier = uniqueIdSupplier;
  }

  public void visitLogicalAssignmentOperator(NodeTraversal t, Node logicalAssignment) {
    Node enclosingStatement = NodeUtil.getEnclosingStatement(logicalAssignment);

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

    Node replacement;

    while (left.isCast()) {
      // This pass runs after type checking, which is what requires the CAST nodes.
      // They aren't needed after that, but they are still in the AST until
      // Normalization removes them. So, it's safe to just remove this CAST now to simplify things.
      // If this pass moves after Normalization, then it will no longer be necessary to check for
      // CAST nodes.
      left = left.removeFirstChild();
    }

    if (left.isName()) {
      replacement = handleLHSName(logicalAssignment, left, right);

    } else {
      checkState(left.isGetProp() || left.isGetElem(), left);
      replacement =
          handleLHSPropertyReference(t, logicalAssignment, left, right, enclosingStatement);
    }

    logicalAssignment.replaceWith(replacement);

    compiler.reportChangeToEnclosingScope(enclosingStatement);
  }

  public Node handleLHSName(Node logicalAssignment, Node left, Node right) {
    // handle name case
    // e.g. convert `name ??= something()` to
    // `name ?? (name = something())`
    Node assignToRHS = astFactory.createAssign(left, right).srcref(right);

    // TODO(bradfordcsmith): We should have an AstFactory method for this.
    return new Node(
            NodeUtil.getOpFromAssignmentOp(logicalAssignment), left.cloneNode(), assignToRHS)
        .copyTypeFrom(logicalAssignment)
        .srcref(logicalAssignment);
  }

  public Node handleLHSPropertyReference(
      NodeTraversal t, Node logicalAssignment, Node left, Node right, Node enclosingStatement) {
    // handle getprop case and getelem case
    CompilerInput input = t.getInput();
    String uniqueId = uniqueIdSupplier.getUniqueId(input);
    String tempVarName = TEMP_VAR_NAME_PREFIX + uniqueId;

    Node assignToRHS;
    Node newLHS;

    Node objectNode = left.removeFirstChild();

    Node let = astFactory.createSingleLetNameDeclaration(tempVarName).srcrefTree(logicalAssignment);
    let.insertBefore(enclosingStatement);

    Node tempName = astFactory.createName(tempVarName, type(objectNode)).srcref(objectNode);
    Node assignTemp =
        astFactory.createAssign(tempName, objectNode).srcref(objectNode); // (tmp = someExpression)

    if (left.isGetProp()) {
      // handle getprop case
      // e.g. convert `(someExpression).property ??= something()` to
      // let tmp;
      // (tmp = someExpression).property ?? (tmp.property = something());
      String propertyName = left.getString();

      Node tempProp =
          astFactory
              .createGetProp(tempName.cloneNode(), propertyName, type(left))
              .srcref(right); // (tmp.property)
      assignToRHS =
          astFactory.createAssign(tempProp, right).srcref(right); // (tmp.property = something())
      newLHS =
          astFactory
              .createGetProp(assignTemp, propertyName, type(left))
              .srcref(left); // ((tmp = someExpression).property)
    } else {
      // handle getelem case
      // e.g. convert `someExpression[indexExpression] ??= something()` to
      // let tmp;
      // let tmpIndex;
      // (tmp = someExpression)[tmpIndex = indexExpression] ?? (tmp[tmpIndex] = something());
      checkState(left.isGetElem(), left);
      String tempIndexVarName = TEMP_INDEX_VAR_NAME_PREFIX + uniqueId;

      Node indexExprNode = left.getLastChild().detach();

      Node letIndex =
          astFactory.createSingleLetNameDeclaration(tempIndexVarName).srcrefTree(logicalAssignment);
      letIndex.insertBefore(enclosingStatement);

      Node tempIndexName =
          astFactory
              .createName(tempIndexVarName, type(indexExprNode))
              .srcref(indexExprNode); // tmpIndex

      Node assignTempIndex =
          astFactory
              .createAssign(tempIndexName, indexExprNode)
              .srcref(indexExprNode); // [tmpIndex = indexExpression]

      Node tempElem =
          astFactory
              .createGetElem(tempName.cloneNode(), tempIndexName.cloneNode())
              .copyTypeFrom(left)
              .srcref(right); // (tmp[tmpIndex])

      assignToRHS =
          astFactory.createAssign(tempElem, right).srcref(right); // (tmp[tmpIndex] = something())
      newLHS =
          astFactory
              .createGetElem(assignTemp, assignTempIndex)
              .copyTypeFrom(left)
              .srcref(left); // (tmp = someExpression)[tmpIndex = indexExpression]
    }

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

    // TODO(bradfordcsmith): We should have an AstFactory method for this.
    return new Node(NodeUtil.getOpFromAssignmentOp(logicalAssignment), newLHS, assignToRHS)
        .copyTypeFrom(logicalAssignment)
        .srcref(logicalAssignment);
  }
}