function isNaN(value) {
    // `NaN` as a primitive is the only value that is not equal to itself
    // (perform the [[Class]] check first to avoid errors with some host objects in IE)
    return toString.call(value) == numberClass && value != +value
  }
Checks if `value` is `NaN`.

Note: This is not the same as native `isNaN`, which will return true for
`undefined` and other values. See http://es5.github.com/#x15.1.2.4.

@deprecated
@static
@memberOf _
@category Objects
@param {Mixed} value The value to check.
@returns {Boolean} Returns `true` if the `value` is `NaN`, else `false`.
@example

_.isNaN(NaN);
// => true

_.isNaN(new Number(NaN));
// => true

isNaN(undefined);
// => true

_.isNaN(undefined);
// => false