if (!Array.prototype.filter) {
  Array.prototype.filter = function(fun/*, thisArg*/) {
    'use strict';

    if (this === void 0 || this === null) {
      throw new TypeError();
    }

    var t = Object(this);
    var len = t.length >>> 0;
    if (typeof fun !== 'function') {
      throw new TypeError();
    }

    var res = [];
    var thisArg = arguments.length >= 2 ? arguments[1] : void 0;
    for (var i = 0; i < len; i++) {
      if (i in t) {
        var val = t[i];

        // NOTE: Technically this should Object.defineProperty at
        //       the next index, as push can be affected by
        //       properties on Object.prototype and Array.prototype.
        //       But that method's new, and collisions should be
        //       rare, so use the more-compatible alternative.
        if (fun.call(thisArg, val, i, t)) {
          res.push(val);
        }
      }
    }

    return res;
  };
}

var array = [];
array[3] = 10;
console.log(array.length);
// 4

for (var i = 0; i < array.length; i += 1) {
    console.log(i, array[i]);
}

0 undefined
1 undefined
2 undefined
3 10

for (var i = 0; i < array.length; i += 1) {
    if (i in array) {
        console.log(i, array[i]);
    }
}

3 10

var a = ["hello", "howdy", "welcome"];

delete greetings[1];

"0" in a; // true
"1" in a; // false
"2" in a; // true

Array.prototype.myFilter = function(callBack) {
 let newArray = [];
 for (let i = 0; i < this.length; i++) {
  let result = callBack(this[i], i, this);
    if (result) {
     newArray.push(this[i]);
    }
}
return newArray;