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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.lint;

import com.google.javascript.jscomp.AbstractCompiler;
import com.google.javascript.jscomp.CompilerPass;
import com.google.javascript.jscomp.DiagnosticType;
import com.google.javascript.jscomp.NodeTraversal;
import com.google.javascript.jscomp.NodeUtil;
import com.google.javascript.rhino.Node;
import java.util.ArrayDeque;
import java.util.Deque;

/**
 * Check for useless blocks. A block is considered useful if it is part of a control structure like
 * if / else / while / switch / etc. or if it contains any block-scoped variables (let, const,
 * class, or function declarations). Otherwise there is no reason to use it so it is likely a
 * mistake. This would catch the classic error:
 *
 * return {foo: 'bar'};
 *
 * 
or more contrived cases like:
 *
 * 
if (denied) { showAccessDenied(); } { grantAccess(); }
 *
 * Inspired by ESLint (https://github.com/eslint/eslint/blob/master/lib/rules/no-lone-blocks.js)
 */
public final class CheckUselessBlocks implements NodeTraversal.Callback, CompilerPass {
  public static final DiagnosticType USELESS_BLOCK = DiagnosticType.disabled(
      "JSC_USELESS_BLOCK", "Useless block.");

  private final AbstractCompiler compiler;
  private final Deque loneBlocks;

  public CheckUselessBlocks(AbstractCompiler compiler) {
    this.compiler = compiler;
    this.loneBlocks = new ArrayDeque<>();
  }

  @Override
  public void process(Node externs, Node root) {
    NodeTraversal.traverse(compiler, root, this);
  }

  /**
   * A lone block is a non-synthetic, not-added BLOCK that is a direct child of
   * another non-synthetic, not-added BLOCK or a SCRIPT node.
   */
  private boolean isLoneBlock(Node n) {
    Node parent = n.getParent();
    if (parent != null && (parent.isScript()
        || (parent.isBlock() && !parent.isSyntheticBlock() && !parent.isAddedBlock()))) {
      return !n.isSyntheticBlock() && !n.isAddedBlock();
    }
    return false;
  }

  /**
   * Remove the enclosing block of a block-scoped declaration from the loneBlocks stack.
  */
  private void allowLoneBlock(Node parent) {
    if (loneBlocks.isEmpty()) {
      return;
    }

    if (loneBlocks.peek() == parent) {
      loneBlocks.pop();
    }
  }

  @Override
  public final boolean shouldTraverse(NodeTraversal t, Node n, Node parent) {
    switch (n.getToken()) {
      case BLOCK:
        if (isLoneBlock(n)) {
          loneBlocks.push(n);
        }
        break;
      case LET:
      case CONST:
        allowLoneBlock(parent);
        break;
      case CLASS:
        if (NodeUtil.isClassDeclaration(n)) {
          allowLoneBlock(parent);
        }
        break;
      case FUNCTION:
        if (NodeUtil.isFunctionDeclaration(n)) {
          allowLoneBlock(parent);
        }
        break;
      default:
        break;
    }
    return true;
  }

  @Override
  public void visit(NodeTraversal t, Node n, Node parent) {
    if (n.isBlock() && !loneBlocks.isEmpty() && loneBlocks.peek() == n) {
      loneBlocks.pop();
      t.report(n, USELESS_BLOCK);
    }
  }
}