hexo/node_modules/core-js/modules/_ctx.js

// optional / simple context binding
var aFunction = require('./_a-function');
module.exports = function (fn, that, length) {
  aFunction(fn);
  if (that === undefined) return fn;
  switch (length) {
    case 1: return function (a) {
      return fn.call(that, a);
    };
    case 2: return function (a, b) {
      return fn.call(that, a, b);
    };
    case 3: return function (a, b, c) {
      return fn.call(that, a, b, c);
    };
  }
  return function (/* ...args */) {
    return fn.apply(that, arguments);
  };
};