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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 2008 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.base.Predicate;
import com.google.javascript.jscomp.ControlFlowGraph.Branch;
import com.google.javascript.jscomp.base.Tri;
import com.google.javascript.jscomp.graph.GraphNode;
import com.google.javascript.jscomp.graph.GraphReachability;
import com.google.javascript.jscomp.graph.GraphReachability.EdgeTuple;
import com.google.javascript.rhino.Node;

/**
 * Use {@link ControlFlowGraph} and {@link GraphReachability} to inform user about unreachable code.
 */
class CheckUnreachableCode extends NodeTraversal.AbstractCfgCallback {

  static final DiagnosticType UNREACHABLE_CODE = DiagnosticType.warning(
      "JSC_UNREACHABLE_CODE", "unreachable code");

  private final AbstractCompiler compiler;

  CheckUnreachableCode(AbstractCompiler compiler) {
    this.compiler = compiler;
  }

  @Override
  public void enterScopeWithCfg(NodeTraversal t) {
    if (NodeUtil.isValidCfgRoot(t.getScopeRoot())) {
      initScope(getControlFlowGraph(compiler));
    }
  }

  @Override
  public boolean shouldTraverse(NodeTraversal t, Node n, Node parent) {
    ControlFlowGraph cfg = getControlFlowGraph(compiler);
    GraphNode gNode = cfg.getNode(n);
    if (gNode != null && gNode.getAnnotation() != GraphReachability.REACHABLE) {

      // Only report error when there are some line number informations.
      // There are synthetic nodes with no line number informations, nodes
      // introduce by other passes (although not likely since this pass should
      // be executed early) or some rhino bug.
      if (n.getLineno() != -1 &&
          // Allow spurious semi-colons and spurious breaks.
          !n.isEmpty() && !n.isBreak()) {
        compiler.report(JSError.make(n, UNREACHABLE_CODE));
        // From now on, we are going to assume the user fixed the error and not
        // give more warning related to code section reachable from this node.
        new GraphReachability<>(cfg).recompute(n);

        // Saves time by not traversing children.
        return false;
      }
    }
    return true;
  }

  private void initScope(ControlFlowGraph controlFlowGraph) {
    new GraphReachability<>(controlFlowGraph, REACHABLE)
        .compute(controlFlowGraph.getEntry().getValue());
  }

  private static final Predicate> REACHABLE =
      (EdgeTuple input) -> {
        Branch branch = input.edge;
        if (!branch.isConditional()) {
          return true;
        }
        Node predecessor = input.sourceNode;
        Node condition = NodeUtil.getConditionExpression(predecessor);

        // TODO(user): Handle more complicated expression like true == true,
        // etc....
        if (condition != null) {
          Tri val = NodeUtil.getBooleanValue(condition);
          if (val != Tri.UNKNOWN) {
            return val.toBoolean(true) == (branch == Branch.ON_TRUE);
          }
        }
        return true;
      };
}