filter-map-reduce.js

// filter
Array.prototype.myFilter = function(func) {
  const arr = []

  for (let i = 0; i < this.length; i++) {
    const item = this[i]

    if(func(item, i, this)) {
      arr.push(item)
    }
  }

  return arr
}

// map
Array.prototype.myMap = function(func) {
  const arr = []

  for (let i = 0; i < this.length; i++) {
    arr.push(func(this[i], i , this))   
  }

  return arr
}

// reduce
Array.prototype.myReduce = function(func, startValue) {
  const arr = arguments.length === 1 
    ? this 
    : [startValue, ...this]

  let accumulator = arr[0]

  for (let i = 1; i < arr.length; i++) {
    accumulator = func(accumulator, arr[i], i, this) 
  }

  return accumulator
}