用 reduce 实现 map 的功能

Array.prototype.map = function (callback) {
  const array = this;
  return array.reduce((acc, cur, index) => {
    acc.push(callback(cur, index, array));
    return acc;
  }, []);
};

测试：

var m = [1, 2, 3, 4, 5].map(function (v, i, arr) {
  return v + v;
});
console.log(m);

reduce案例

var array = [
  {
    selector: "sss",
    rules: "rrrr",
  },
  {
    selector: "sss2",
    rules: "rrr3",
  },
];

function transform(array, key, value) {
  return array.reduce((obj, item) => {
    obj[item[key]] = (obj[item[key]] || []).concat(item[value]);
    return obj;
  }, {});
}

var tree = transform(array, "selector", "rules");

console.log(tree);