quivalence is
  // determined by having the same number of owned properties (as verified
  // with Object.prototype.hasOwnProperty.call), the same set of keys
  // (although not necessarily the same order), equivalent values for every
  // corresponding key, and an identical 'prototype' property. Note: this
  // accounts for both named and indexed properties on Arrays.
  return objEquiv(a, b);
}

/**
 * Tests to see if an object is a particular instance of the given type.
 *
 * @param {object} object
 * @param {string} type
 */
function instanceOf(object, type) {
  return Object.prototype.toString.call(object) == "[object " + type + "]";
}

/**
 * Checks is see if the value is undefined or null.
 *
 * @param {any} value
 */
function isUndefinedOrNull(value) {
  return value === null || value === undefined;
}

/**
 * Checks to see if the object is an arguments object.
 *
 * @param {object} object
 */
function isArguments(object) {
  return instanceOf(object, "Arguments");
}

/**
 * Compares objects for equivalence.
 *
 * @param {object} a
 * @param {object} b
 * @returns {boolean}
 */
function objEquiv(a, b) {
  if (isUndefinedOrNull(a) || isUndefinedOrNull(b)) {
    return false;
  }
  // An identical 'prototype' property.
  if ((a.prototype || undefined) != (b.prototype || undefined)) {
    return false;
  }

  // Check for ArrayBuffer equality
  if (instanceOf(a, "ArrayBuffer") && instanceOf(b, "ArrayBuffer")) {
    if (a.byteLength !== b.byteLength) {
      return false;
    }
    const viewA = new Uint8Array(a);
    const viewB = new Uint8Array(b);
    for (let i = 0; i < viewA.length; i++) {
      if (viewA[i] !== viewB[i]) {
        return false;
      }
    }
    return true;
  }

  // Object.keys may be broken through screwy arguments passing. Converting to
  // an array solves the problem.
  if (isArguments(a)) {
    if (!isArguments(b)) {
      return false;
    }
    a = pSlice.call(a);
    b = pSlice.call(b);
    return _deepEqual(a, b);
  }
  let ka, kb;
  try {
    ka = Object.keys(a);
    kb = Object.keys(b);
  } catch (e) {
    // Happens when one is a string literal and the other isn't
    return false;
  }
  // Having the same number of owned properties (keys incorporates
  // hasOwnProperty)
  if (ka.length != kb.length) {
    return false;
  }
  // The same set of keys (although not necessarily the same order),
  ka.sort();
  kb.sort();
  // Equivalent values for every corresponding key, and possibly expensive deep
  // test
  for (let key of ka) {
    if (!_deepEqual(a[key], b[key])) {
      return false;
    }
  }
  return true;
}

// ... End of previously MIT-licensed code.
PK