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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 2016 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 com.google.javascript.rhino.IR;
import com.google.javascript.rhino.Node;

/** An optimization pass to re-write J2CL Equality.$same. */
public class J2clEqualitySameRewriterPass extends AbstractPeepholeOptimization {

  /** Whether to use "==" or "===". */
  private static enum Eq {
    DOUBLE,
    TRIPLE
  }

  private boolean shouldRunJ2clPasses = false;

  @Override
  void beginTraversal(AbstractCompiler compiler) {
    super.beginTraversal(compiler);
    shouldRunJ2clPasses = J2clSourceFileChecker.shouldRunJ2clPasses(compiler);
  }

  @Override
  Node optimizeSubtree(Node node) {
    if (!shouldRunJ2clPasses) {
      return node;
    }

    if (!isEqualitySameCall(node)) {
      return node;
    }

    Node replacement = trySubstituteEqualitySame(node);
    if (replacement != node) {
      replacement = replacement.useSourceInfoIfMissingFrom(node);
      node.replaceWith(replacement);
      compiler.reportChangeToEnclosingScope(replacement);
    }
    return replacement;
  }

  private Node trySubstituteEqualitySame(Node callNode) {
    Node firstExpr = callNode.getSecondChild();
    Node secondExpr = callNode.getLastChild();

    if (NodeUtil.isNullOrUndefined(firstExpr) || NodeUtil.isNullOrUndefined(secondExpr)) {
      // At least one side is null or undefined so no coercion danger.
      return rewriteToEq(firstExpr, secondExpr, Eq.DOUBLE);
    }

    if (NodeUtil.isLiteralValue(firstExpr, true) || NodeUtil.isLiteralValue(secondExpr, true)) {
      // There is a coercion danger but since at least one side is not null, we can use === that
      // will not trigger any coercion.
      return rewriteToEq(firstExpr, secondExpr, Eq.TRIPLE);
    }

    return callNode;
  }

  private Node rewriteToEq(Node firstExpr, Node secondExpr, Eq eq) {
    firstExpr.detach();
    secondExpr.detach();
    return eq == Eq.DOUBLE ? IR.eq(firstExpr, secondExpr) : IR.sheq(firstExpr, secondExpr);
  }

  private static boolean isEqualitySameCall(Node node) {
    return node.isCall()
        // Do not optimize if one or both parameters were removed
        && node.hasXChildren(3)
        && isEqualitySameMethodName(node.getFirstChild());
  }

  private static boolean isEqualitySameMethodName(Node fnName) {
    if (!fnName.isQualifiedName()) {
      return false;
    }
    // NOTE: This should be rewritten to use method name + file name of definition site
    // like other J2CL passes, which is more precise.
    String originalQname = fnName.getOriginalQualifiedName();
    return originalQname.endsWith(".$same") && originalQname.contains("Equality");
  }
}