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

import com.google.common.base.Suppliers;
import com.google.javascript.jscomp.colors.StandardColors;
import com.google.javascript.jscomp.parsing.parser.FeatureSet;
import com.google.javascript.jscomp.parsing.parser.FeatureSet.Feature;
import com.google.javascript.rhino.Node;
import java.util.function.Supplier;

/** Replaces the ES7 `**` and `**=` operators to calls to `Math.pow`. */
public final class Es7RewriteExponentialOperator implements NodeTraversal.Callback, CompilerPass {

  static final DiagnosticType TRANSPILE_EXPONENT_USING_BIGINT =
      DiagnosticType.error(
          "JSC_TRANSPILE_EXPONENT_USING_BIGINT",
          "Cannot transpile `**` operator applied to BigInt operands.");

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

  private final AbstractCompiler compiler;
  private final AstFactory astFactory;
  private final UniqueIdSupplier uniqueIdSupplier;
  // This node should only ever be cloned, not directly inserted.
  private final Supplier mathPowCall = Suppliers.memoize(this::createMathPowCall);

  public Es7RewriteExponentialOperator(AbstractCompiler compiler) {
    this.compiler = compiler;
    this.astFactory = compiler.createAstFactory();
    this.uniqueIdSupplier = compiler.getUniqueIdSupplier();
  }

  @Override
  public void process(Node externs, Node root) {
    NodeTraversal.traverse(compiler, root, this);
    TranspilationPasses.maybeMarkFeatureAsTranspiledAway(compiler, root, Feature.EXPONENT_OP);
  }

  @Override
  public boolean shouldTraverse(NodeTraversal t, Node n, Node parent) {
    if (n.isScript()) {
      FeatureSet scriptFeatures = NodeUtil.getFeatureSetOfScript(n);
      // Runs only if features exist in the script
      return scriptFeatures == null || scriptFeatures.contains(Feature.EXPONENT_OP);
    }
    return true;
  }

  @Override
  public void visit(NodeTraversal t, Node n, Node parent) {
    switch (n.getToken()) {
      case EXPONENT:
        if (checkOperatorType(n)) {
          visitExponentiationOperator(n);
        }
        break;
      case ASSIGN_EXPONENT:
        if (checkOperatorType(n)) {
          visitExponentiationAssignmentOperator(t, n);
        }
        break;
      default:
        break;
    }
  }

  private void visitExponentiationOperator(Node operator) {
    Node callClone = mathPowCall.get().cloneTree();
    callClone.addChildToBack(operator.removeFirstChild()); // Base argument.
    callClone.addChildToBack(operator.removeFirstChild()); // Exponent argument.

    callClone.srcrefTreeIfMissing(operator);
    operator.replaceWith(callClone);

    compiler.reportChangeToEnclosingScope(callClone);
  }

  private void visitExponentiationAssignmentOperator(NodeTraversal t, Node operator) {
    Node enclosingStatement = NodeUtil.getEnclosingStatement(operator);

    Node left = operator.removeFirstChild();

    Node replacement;

    if (left.isName()) {
      replacement = handleLHSName(operator, left);
    } else {
      checkState(left.isGetProp() || left.isGetElem(), left);
      replacement = handleLHSPropertyReference(t, operator, left, enclosingStatement);
    }

    operator.replaceWith(replacement);

    compiler.reportChangeToEnclosingScope(enclosingStatement);
  }

  private Node handleLHSName(Node operator, Node left) {
    // handle name case
    // e.g. convert `name **= value` to
    // `name = Math.pow(name, value)`
    Node callClone = mathPowCall.get().cloneTree();
    callClone.addChildToBack(left.cloneTree()); // Base argument.
    callClone.addChildToBack(operator.removeFirstChild()); // Exponent argument.

    return astFactory.createAssign(left, callClone).srcrefTreeIfMissing(operator);
  }

  private Node handleLHSPropertyReference(
      NodeTraversal t, Node operator, Node left, 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 newLHS;
    Node tempPropOrElem;

    Node objectNode = left.removeFirstChild();

    Node let = astFactory.createSingleLetNameDeclaration(tempVarName).srcrefTree(operator);
    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 **= value` to
      // let tmp;
      // (tmp = someExpression).property = Math.pow(tmp.property, value);
      String propertyName = left.getString();

      tempPropOrElem =
          astFactory
              .createGetProp(tempName.cloneNode(), propertyName, type(left))
              .srcref(left); // (tmp.property)
      newLHS =
          astFactory
              .createGetProp(assignTemp, propertyName, type(left))
              .srcref(left); // ((tmp = someExpression).property)
    } else {
      // handle getelem case
      // e.g. convert `someExpression[indexExpression] **= value` to
      // let tmp;
      // let tmpIndex;
      // `(tmp = someExpression)[(tmpIndex = indexExpression)] = Math.pow(tmp[tmpIndex], value);`
      checkState(left.isGetElem(), left);
      String tempIndexVarName = TEMP_INDEX_VAR_NAME_PREFIX + uniqueId;

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

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

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

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

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

      newLHS =
          astFactory
              .createGetElem(assignTemp, assignTempIndex)
              .copyTypeFrom(left)
              .srcref(left); // (tmp = someExpression)[tmpIndex = indexExpression]
    }
    Node callClone = mathPowCall.get().cloneTree();
    callClone.addChildToBack(tempPropOrElem.cloneTree()); // Base argument.
    callClone.addChildToBack(operator.removeFirstChild()); // Exponent argument.

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

    return astFactory.createAssign(newLHS, callClone).srcrefTreeIfMissing(operator);
  }

  private Node createMathPowCall() {
    return astFactory.createCall(
        astFactory.createQName(compiler.getTranspilationNamespace(), "Math.pow"),
        type(StandardColors.NUMBER));
  }

  // Report an error if the `**` getting transpiled to `Math.pow()`is of BIGINT type
  private boolean checkOperatorType(Node operator) {
    checkArgument(operator.isExponent() || operator.isAssignExponent(), operator);
    if (compiler.hasTypeCheckingRun()) {
      if (operator.getColor().equals(StandardColors.BIGINT)) {
        compiler.report(JSError.make(operator, TRANSPILE_EXPONENT_USING_BIGINT));
        return false;
      }
    }
    return true;
  }
}