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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 2015 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 static com.google.javascript.rhino.jstype.JSTypeNative.ARRAY_TYPE;

import com.google.common.base.Preconditions;
import com.google.common.collect.ImmutableSet;
import com.google.javascript.jscomp.AbstractCompiler;
import com.google.javascript.jscomp.DiagnosticType;
import com.google.javascript.jscomp.HotSwapCompilerPass;
import com.google.javascript.jscomp.NodeTraversal;
import com.google.javascript.jscomp.NodeUtil;
import com.google.javascript.rhino.Node;
import com.google.javascript.rhino.jstype.JSType;
import com.google.javascript.rhino.jstype.JSTypeRegistry;
import com.google.javascript.rhino.jstype.TemplatizedType;

import java.util.Set;

/**
 * Checks when the pattern for (x in arr) { ... } where arr is an array,
 * or an union type containing an array. Also checks several goog.object methods
 * that are essentially for/in loops.
 *
 */
public final class CheckForInOverArray extends NodeTraversal.AbstractPostOrderCallback
    implements HotSwapCompilerPass {
  final AbstractCompiler compiler;
  final JSTypeRegistry typeRegistry;

  private static final Set GOOG_OBJECT_METHODS =
      ImmutableSet.of(
          "goog.object.forEach",
          "goog.object.filter",
          "goog.object.map",
          "goog.object.some",
          "goog.object.every",
          "goog.object.getCount",
          "goog.object.getAnyKey",
          "goog.object.getAnyValue",
          "goog.object.contains",
          "goog.object.getValues",
          "goog.object.getKeys",
          "goog.object.findKey",
          "goog.object.findValue",
          "goog.object.isEmpty",
          "goog.object.clear",
          "goog.object.remove",
          "goog.object.equals",
          "goog.object.clone",
          "goog.object.unsafeClone",
          "goog.object.transpose");

  public static final DiagnosticType FOR_IN_OVER_ARRAY =
      DiagnosticType.warning(
          "JSC_FOR_IN_OVER_ARRAY",
          "For..in over array is discouraged.");

  public static final DiagnosticType ARRAY_PASSED_TO_GOOG_OBJECT =
      DiagnosticType.warning(
          "JSC_ARRAY_PASSED_TO_GOOG_OBJECT",
          "{0} expects an object, not an array. Did you mean to use goog.array?");

  public CheckForInOverArray(AbstractCompiler compiler) {
    this.compiler = compiler;
    this.typeRegistry = compiler.getTypeRegistry();
  }

  public boolean isForInOverArray(Node n) {
    if (NodeUtil.isForIn(n)) {
      Preconditions.checkState(n.getChildCount() == 3, n);
      // get the second child, which represents
      // B in construct "for (A in B) { C }"
      Node child = n.getSecondChild();
      JSType type = child.getJSType();
      if (type != null && containsArray(type)) {
        return true;
      }
    }
    return false;
  }

  public boolean isGoogObjectIterationOverArray(Node n) {
    if (!n.isCall()) {
      return false;
    }
    if (!n.getFirstChild().isQualifiedName()) {
      return false;
    }

    String name = n.getFirstChild().getQualifiedName();
    if (!GOOG_OBJECT_METHODS.contains(name)) {
      return false;
    }

    Node firstArg = n.getSecondChild();
    if (firstArg == null) {
      return false;
    }
    JSType type = firstArg.getJSType();
    return type != null && containsArray(type);
  }

  private boolean isArray(JSType type) {
    if (type.isArrayType()) {
      return true;
    }
    TemplatizedType templatizedType = type.toMaybeTemplatizedType();
    return templatizedType != null && templatizedType.getReferencedType().isArrayType();
  }

  private boolean containsArray(JSType type) {
    if (isArray(type)) {
      return true;
    }
    if (type.isUnionType()) {
      JSType arrayType = typeRegistry.getNativeType(ARRAY_TYPE);
      for (JSType alternate : type.toMaybeUnionType().getAlternates()) {
        if (alternate.isSubtype(arrayType)) {
          return true;
        }
      }
    }
    return false;
  }

  @Override
  public void visit(NodeTraversal t, Node n, Node parent) {
    if (isForInOverArray(n)) {
      compiler.report(t.makeError(n, FOR_IN_OVER_ARRAY));
    } else if (isGoogObjectIterationOverArray(n)) {
      compiler.report(
          t.makeError(n, ARRAY_PASSED_TO_GOOG_OBJECT, n.getFirstChild().getQualifiedName()));
    }
  }

  @Override
  public void process(Node externs, Node root) {
    NodeTraversal.traverseEs6(compiler, root, this);
  }

  @Override
  public void hotSwapScript(Node scriptRoot, Node originalRoot) {
    NodeTraversal.traverseEs6(compiler, scriptRoot, this);
  }
}