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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 2020 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.jscomp.OptionalChainRewriter.TmpVarNameCreator;
import com.google.javascript.jscomp.parsing.parser.FeatureSet;
import com.google.javascript.jscomp.parsing.parser.FeatureSet.Feature;
import com.google.javascript.rhino.IR;
import com.google.javascript.rhino.Node;
import java.util.ArrayList;
import java.util.function.Function;

/** Replaces the ES2020 `?.` operator with conditional (? :). */
final class RewriteOptionalChainingOperator implements CompilerPass {

  private final AbstractCompiler compiler;

  /**
   * Produces the object responsible for creating temporary variable names for a given
   * CompilerInput.
   */
  private final Function getTmpVarNameCreatorForInput;

  /** Constructor to be used for actual transpilation */
  RewriteOptionalChainingOperator(AbstractCompiler compiler) {
    this.compiler = compiler;
    // Temporary variable names are generated based on the compiler input they will go into and use
    // a recognizable prefix to aid debugging.
    final UniqueIdSupplier uniqueIdSupplier = compiler.getUniqueIdSupplier();
    this.getTmpVarNameCreatorForInput =
        (CompilerInput input) -> () -> "$jscomp$optchain$tmp" + uniqueIdSupplier.getUniqueId(input);
  }

  /** Constructor for testing. */
  RewriteOptionalChainingOperator(AbstractCompiler compiler, TmpVarNameCreator tmpVarNameCreator) {
    this.compiler = compiler;
    // The test provides a simple variable name creator that makes for more readable test cases.
    this.getTmpVarNameCreatorForInput = (CompilerInput input) -> tmpVarNameCreator;
  }

  @Override
  public void process(Node externs, Node root) {
    NodeTraversal.traverseRoots(compiler, new ArrowRewriteCallBack(), externs, root);
    NodeTraversal.traverseRoots(compiler, new TranspilationCallback(), externs, root);
    TranspilationPasses.maybeMarkFeaturesAsTranspiledAway(compiler, Feature.OPTIONAL_CHAINING);
  }

  /** Locates and transpiles all optional chains. */
  private class TranspilationCallback implements NodeTraversal.Callback {
    private final OptionalChainRewriter.Builder rewriterBuilder =
        OptionalChainRewriter.builder(compiler);
    private final ArrayList optChains = new ArrayList<>();

    @Override
    public boolean shouldTraverse(NodeTraversal t, Node n, Node parent) {
      if (n.isScript()) {
        // Set the TmpVarNameCreator to be used when rewriting optional chains in this script.
        rewriterBuilder.setTmpVarNameCreator(getTmpVarNameCreatorForInput.apply(t.getInput()));
        FeatureSet scriptFeatures = NodeUtil.getFeatureSetOfScript(n);
        return scriptFeatures == null || scriptFeatures.contains(Feature.OPTIONAL_CHAINING);
      }
      return true;
    }

    @Override
    public void visit(NodeTraversal t, Node n, Node parent) {
      if (NodeUtil.isEndOfFullOptChain(n)) {
        optChains.add(rewriterBuilder.build(n));
      } else if (n.isScript()) {
        // We transpile all of the optional chains in a single script as a batch because,
        // rewriting changes the AST in ways that could interfere with traversal
        // if we change the chains as we visit them.
        if (!optChains.isEmpty()) {
          for (OptionalChainRewriter optChain : optChains) {
            optChain.rewrite();
          }
          optChains.clear();
        }
      }
    }
  }

  /**
   * Optional chaining rewriting declares new variables within the enclosing scope of the chain.
   * This callback makes arrow functions have a block around the body to prevent new declarations
   * from leaking into the higher scope.
   */
  //  TODO(b/197349249) Rewriting arrows would become unnecessary if this pass ran normalized.
  private class ArrowRewriteCallBack implements NodeTraversal.Callback {

    @Override
    public boolean shouldTraverse(NodeTraversal t, Node n, Node parent) {
      if (n.isScript()) {
        FeatureSet scriptFeatures = NodeUtil.getFeatureSetOfScript(n);
        return scriptFeatures == null || scriptFeatures.contains(Feature.OPTIONAL_CHAINING);
      }
      return true;
    }

    @Override
    public void visit(NodeTraversal t, Node n, Node parent) {
      if (!NodeUtil.isEndOfFullOptChain(n)) {
        return;
      }
      Node arrowOrBlock =
          NodeUtil.getEnclosingNode(n, (node) -> node.isArrowFunction() || node.isBlock());
      if (arrowOrBlock != null && arrowOrBlock.isArrowFunction()) {
        if (!NodeUtil.getFunctionBody(arrowOrBlock).isBlock()) {
          // create a block around the arrow function body.
          Node returnValue = NodeUtil.getFunctionBody(arrowOrBlock);
          Node body = IR.block(IR.returnNode(returnValue.detach()));
          body.srcrefTreeIfMissing(returnValue);
          arrowOrBlock.addChildToBack(body);
          compiler.reportChangeToEnclosingScope(body);
        }
      }
    }
  }
}