function equals(o1, o2) {
  if (o1 === o2) return true;
  if (o1 === null || o2 === null) return false;
  // eslint-disable-next-line no-self-compare
  if (o1 !== o1 && o2 !== o2) return true; // NaN === NaN
  var t1 = typeof o1, t2 = typeof o2, length, key, keySet;
  if (t1 === t2 && t1 === 'object') {
    if (isArray(o1)) {
      if (!isArray(o2)) return false;
      if ((length = o1.length) === o2.length) {
        for (key = 0; key < length; key++) {
          if (!equals(o1[key], o2[key])) return false;
        }
        return true;
      }
    } else if (isDate(o1)) {
      if (!isDate(o2)) return false;
      return simpleCompare(o1.getTime(), o2.getTime());
    } else if (isRegExp(o1)) {
      if (!isRegExp(o2)) return false;
      return o1.toString() === o2.toString();
    } else {
      if (isScope(o1) || isScope(o2) || isWindow(o1) || isWindow(o2) ||
        isArray(o2) || isDate(o2) || isRegExp(o2)) return false;
      keySet = createMap();
      for (key in o1) {
        if (key.charAt(0) === '$' || isFunction(o1[key])) continue;
        if (!equals(o1[key], o2[key])) return false;
        keySet[key] = true;
      }
      for (key in o2) {
        if (!(key in keySet) &&
            key.charAt(0) !== '$' &&
            isDefined(o2[key]) &&
            !isFunction(o2[key])) return false;
      }
      return true;
    }
  }
  return false;
}
@ngdoc function
@name angular.equals
@module ng
@kind function

@description
Determines if two objects or two values are equivalent. Supports value types, regular
expressions, arrays and objects.

Two objects or values are considered equivalent if at least one of the following is true:

* Both objects or values pass `===` comparison.
* Both objects or values are of the same type and all of their properties are equal by
  comparing them with `angular.equals`.
* Both values are NaN. (In JavaScript, NaN == NaN => false. But we consider two NaN as equal)
* Both values represent the same regular expression (In JavaScript,
  /abc/ == /abc/ => false. But we consider two regular expressions as equal when their textual
  representation matches).

During a property comparison, properties of `function` type and properties with names
that begin with `$` are ignored.

Scope and DOMWindow objects are being compared only by identify (`===`).

@param {*} o1 Object or value to compare.
@param {*} o2 Object or value to compare.
@returns {boolean} True if arguments are equal.

@example
   <example module="equalsExample" name="equalsExample">
     <file name="index.html">
      <div ng-controller="ExampleController">
        <form novalidate>
          <h3>User 1</h3>
          Name: <input type="text" ng-model="user1.name">
          Age: <input type="number" ng-model="user1.age">

          <h3>User 2</h3>
          Name: <input type="text" ng-model="user2.name">
          Age: <input type="number" ng-model="user2.age">

          <div>
            <br/>
            <input type="button" value="Compare" ng-click="compare()">
          </div>
          User 1: <pre>{{user1 | json}}</pre>
          User 2: <pre>{{user2 | json}}</pre>
          Equal: <pre>{{result}}</pre>
        </form>
      </div>
    </file>
    <file name="script.js">
        angular.module('equalsExample', []).controller('ExampleController', ['$scope', function($scope) {
          $scope.user1 = {};
          $scope.user2 = {};
          $scope.compare = function() {
            $scope.result = angular.equals($scope.user1, $scope.user2);
          };
        }]);
    </file>
  </example>