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

/** An optimization pass to rewrite J2CL String.valueOf. */
final class J2clStringValueOfRewriterPass extends AbstractPeepholeOptimization {

  private static final String METHOD_NAME =
      "module$exports$java$lang$String$impl.m_valueOf__java_lang_Object__java_lang_String";

  private boolean shouldRunJ2clPasses = false;

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

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

    Node replacement = tryRewriteStringValueOfCall(node);
    if (replacement != node) {
      replacement = replacement.srcrefIfMissing(node);
      node.replaceWith(replacement);
      reportChangeToEnclosingScope(replacement);
    }
    return replacement;
  }

  /*
   * Cases to rewrite:
   *   String.valueOf("foo") -> String("foo")
   *   String.valueOf(null) -> String("null")
   *   String.valueOf(undefined) -> "null"
   *   String.valueOf("foo" + bar) -> String("foo" + bar)
   *   String.valueOf(bar + "foo") -> String(bar + "foo")
   *   String.valueOf("foo" + String.valueOf(bar)) -> String("foo" + String.valueOf(bar))
   */
  private Node tryRewriteStringValueOfCall(Node n) {
    Node param = n.getSecondChild();
    if (NodeUtil.isUndefined(param)) {
      return IR.string("null");
    } else if (NodeUtil.isDefinedValue(param) && !param.isArrayLit()) {
      // Generate String(param), let other peephole optimizations handle the rest when safe
      return NodeUtil.newCallNode(IR.name("String").srcref(n), param.detach());
    }
    return n;
  }

  private static boolean isStringValueOfCall(Node node) {
    return node.isCall()
        // Do not optimize if the parameter was removed
        && node.hasXChildren(2)
        && isStringValueOfMethodName(node.getFirstChild());
  }

  private static boolean isStringValueOfMethodName(Node fnName) {
    return fnName.isQualifiedName() && fnName.getOriginalQualifiedName().equals(METHOD_NAME);
  }
}