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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.common.collect.ImmutableSet;
import com.google.javascript.jscomp.NodeTraversal.AbstractPostOrderCallback;
import com.google.javascript.rhino.IR;
import com.google.javascript.rhino.Node;

/** A normalization pass to re-write Util.$getDefine calls to make them work in compiled mode. */
public class J2clUtilGetDefineRewriterPass extends AbstractPostOrderCallback
    implements CompilerPass {
  private final AbstractCompiler compiler;
  private ImmutableSet defines;

  public J2clUtilGetDefineRewriterPass(AbstractCompiler compiler) {
    this.compiler = compiler;
  }

  @Override
  public void process(Node externs, Node root) {
    if (!J2clSourceFileChecker.shouldRunJ2clPasses(compiler)) {
      return;
    }
    defines =
        new ProcessDefines.Builder(compiler)
            .setMode(ProcessDefines.Mode.OPTIMIZE)
            .build()
            .collectDefineNames(externs, root);
    NodeTraversal.traverse(compiler, root, this);
  }

  @Override
  public void visit(NodeTraversal t, Node n, Node parent) {
    if (isUtilGetDefineCall(n)) {
      rewriteUtilGetDefine(t, n);
    }
  }

  private void rewriteUtilGetDefine(NodeTraversal t, Node callNode) {
    Node firstExpr = callNode.getSecondChild();
    Node secondExpr = callNode.getLastChild();

    if (secondExpr != firstExpr) {
      secondExpr.detach();
    } else {
      // There is no secondExpr; default to null.
      secondExpr = IR.nullNode();
    }

    Node replacement = getDefineReplacement(firstExpr, secondExpr);
    replacement.srcrefTreeIfMissing(callNode);
    callNode.replaceWith(replacement);
    t.reportCodeChange();
  }

  private Node getDefineReplacement(Node firstExpr, Node secondExpr) {
    if (defines.contains(firstExpr.getString())) {
      Node define = NodeUtil.newQName(compiler, firstExpr.getString());
      Node defineStringValue = NodeUtil.newCallNode(IR.name("String"), define);
      return IR.comma(secondExpr, defineStringValue);
    } else {
      return secondExpr;
    }
  }

  private static boolean isUtilGetDefineCall(Node n) {
    return n.isCall() && isUtilGetDefineMethodName(n.getFirstChild().getQualifiedName());
  }

  private static boolean isUtilGetDefineMethodName(String fnName) {
    // TODO: Switch this to the filename + property name heuristic which is less brittle.
    return fnName != null && fnName.endsWith(".$getDefine") && fnName.contains("Util");
  }
}