function isFunction(value) {
  if (!isObject(value)) {
    return false;
  }
  // The use of `Object#toString` avoids issues with the `typeof` operator
  // in Safari 9 which returns 'object' for typed arrays and other constructors.
  var tag = baseGetTag(value);
  return tag == funcTag || tag == genTag || tag == asyncTag || tag == proxyTag;
}
Checks if `value` is classified as a `Function` object.

@static
@memberOf _
@since 0.1.0
@category Lang
@param {*} value The value to check.
@returns {boolean} Returns `true` if `value` is a function, else `false`.
@example

_.isFunction(_);
// => true

_.isFunction(/abc/);
// => false