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

/**
 * Check for explicit creation of the object equivalents of primitive types (e.g. Boolean instead of
 * boolean) and their use in type declarations.
 *
 * Using these is confusing and gives no benefit. For example, the result of {@code typeof (new
 * Boolean(true))} is {@code "object"}. and the result of {@code (new Boolean(false)) ? "true" :
 * "false"} is {@code "true"}.
 */
public final class CheckPrimitiveAsObject extends AbstractPostOrderCallback
    implements CompilerPass {
  public static final DiagnosticType NEW_PRIMITIVE_OBJECT =
      DiagnosticType.warning("JSC_PRIMITIVE_OBJECT", "Explicit creation of a {0} object.");

  public static final DiagnosticType PRIMITIVE_OBJECT_DECLARATION =
      DiagnosticType.warning(
          "JSC_PRIMITIVE_OBJECT_DECLARATION",
          "Declaration of {0} object instead of primitive type.");

  private static final ImmutableSet PRIMITIVE_OBJECT_CONSTRUCTORS =
      ImmutableSet.of("Boolean", "Number", "String");

  private final AbstractCompiler compiler;

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

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

  @Override
  public void visit(NodeTraversal t, Node n, Node parent) {
    checkForPrimitiveObjectConstructor(t, n);
    checkForPrimitiveObjectDeclaration(t, n);
  }

  private void checkForPrimitiveObjectDeclaration(NodeTraversal t, Node n) {
    JSDocInfo jsDocInfo = n.getJSDocInfo();

    if (jsDocInfo != null) {
      for (Node typeRoot : jsDocInfo.getTypeNodes()) {
        checkTypeNodeForPrimitiveObjectDeclaration(t, typeRoot);
      }
    }
  }

  private void checkTypeNodeForPrimitiveObjectDeclaration(final NodeTraversal t, Node typeRoot) {
    NodeUtil.visitPreOrder(
        typeRoot,
        node -> {
          if (node.isStringLit()) {
            String typeName = node.getString();
            if (PRIMITIVE_OBJECT_CONSTRUCTORS.contains(typeName)) {
              t.report(node, PRIMITIVE_OBJECT_DECLARATION, typeName);
            }
          }
        });
  }

  private void checkForPrimitiveObjectConstructor(NodeTraversal t, Node n) {
    if (n.isNew()) {
      Node constructorFunction = n.getFirstChild();
      if (constructorFunction.isName()) {
        String constructorName = constructorFunction.getString();
        if (PRIMITIVE_OBJECT_CONSTRUCTORS.contains(constructorName)) {
          t.report(n, NEW_PRIMITIVE_OBJECT, constructorName);
        }
      }
    }
  }
}