emulating filter method

myFilter.js

// Array prototype method, however this is not safe, since it is added to the global scope
// definedProperty makes method non-enumerable as well as adding new property to in this case, to Array prototype
Object.defineProperty(Array.prototype, 'myFilter', {
  // if callback is arrow function then context will be set to outer scope regardless of providing context argument
  value: function(callback, context) {
    // Good example for using the context with regular function expression
    // const as = new Set([1, 2, 3]);
    // const bs = new Set([3, 2, 4]);
    // const intersection = [...as].filter(bs.has, bs); // [2, 3]
    if (typeof callback !== 'function') {
      throw new Error(`${callback} is not a function`);
    }

    const length = this.length;
    const newArr = [];

    for (let i = 0; i < length; i++) {
      // check sparse array ex. [1, , ,2]
      if (i in this) {
        if (callback.apply(context, [this[i], i, this])) {
          newArr.push(this[i]);
        }
      }
    }

    return newArr;
  },
});

// class alternative, safe since methods are locally scoped
class MyMethods {
  constructor() {}

  filter(arr, callback, context) {
    if (typeof callback !== 'function') {
      throw new TypeError(`${callback} is not a function`);
    }

    const length = arr.length;
    const newArr = [];

    for (let i = 0; i < length; i++) {
      // check sparse array ex. [1, , ,2]
      if (i in arr) {
        if (callback.apply(context, [arr[i], i, arr])) {
          newArr.push(arr[i]);
        }
      }
    }

    return newArr;
  }
}