JS Functions names with object syntax

Can functions be declared like this in Javascript and safely be used on the web.

$is.array = function (e) {
  return typeof e === "array" || e instanceof Array;
};
$is.element = function (e) {
  return e instanceof Element || e instanceof HTMLDocument;
};
$is.function = function (e) {
  return typeof e === "function";
};

No errors appear to be taking place but I am not familiar with this syntax.