SOURCE

Array.prototype.reduceFaker = function(callback, initialValue) {
  if (this.length === 0 && initialValue === undefined) {
    throw new TypeError('Reduce of empty array with no initial value');
  }

  let accumulator = initialValue !== undefined ? initialValue : this[0];

  for (let i = initialValue !== undefined ? 0 : 1; i < this.length; i++) {
    accumulator = callback(accumulator, this[i], i, this);
  }

  return accumulator;
};

Array.prototype.filter = function(fn, argThis) {
    if(typeof fn !== 'function') {
        throw Error(`${fn}不是一个函数`)
    }
    const filterRes = [];
    for (let i = 0; i < this.length; i++) {
        if (fn.call(argThis, this[i], i, this)) {
            filterRes.push(this[i]);
        }
    }

    return filterRes;
}


let arr = [1, 2, 3];

console.log(arr.reduceFaker((total, curValue) => Number(total) + Number(curValue), 1));

const filterFn = function (item){
    console.log(this);
    return item > 2;
}

// console.log(arr.filter({a: 2}, 2))
console.log(arr.filter(filterFn), 2)
// console.log(arr.filter((item) => item > 2, null))