emulating forEach method

myForEach.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, 'myForEach', {
  // if the callback is an arrow function then context will be set to outer scope regardless of providing context argument
  // On the other hand if callback is a regular function expression, if context is not provided then the 'this'
  // will be set to window(or undefined if strict) since the where the callback function is invoked, it is not an object property,
  // but rather just a function pattern --> https://stackoverflow.com/a/2752318/8234457
  
  // tldr; The 'this' keyword’s value has nothing to do with the function itself, 
  // it's how the function is called determines 'this's value
  value: function(callback, context) {
    if (typeof callback !== 'function') {
      throw new TypeError(`${callback} is not a function`);
    }

    const length = this.length;

    for (let i = 0; i < length; i++) {
      // check sparse arrays(holes i.e. non-existing properties) ex. [1, , ,2]
      if (i in this) {
        callback.apply(context, [this[i], i, this]);
      }
    }
  },
});

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

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

    const length = arr.length;

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