大概是 JS bind 以及 this 的一些尝试

var obj = {
  key: 12,
  log: function () {
    console.log(this.key);
  },
};
obj.log(); /* => 12 */

var log2 = obj.log;
log2(); /* => undefined */

var log3 = obj.log.bind(obj);
log3(); /* => 12 */

/* so */
var $ = document.querySelectorAll.bind(document);

/* > < */

var $ = function () {
  var _args = Array.prototype.slice.call(arguments);
  return Array.prototype.slice.call(
    document.querySelectorAll.apply(document, _args),
  );
};
var $ = function (selector) {
  var els = document.querySelectorAll.bind(document);
  return Array.prototype.slice.call(els(selector));
};