function isUndefined(value) {
      return typeof value == 'undefined';
    }
Checks if `value` is `undefined`.

@static
@memberOf _
@category Lang
@param {*} value The value to check.
@returns {boolean} Returns `true` if `value` is `undefined`, else `false`.
@example

_.isUndefined(void 0);
// => true

_.isUndefined(null);
// => false