NumberOfOccurrences.js

// DESCRIPTION:
// Write a function that returns the number of occurrences of an element in an array.

// This function will be defined as a property of Array with the help of the method Object.defineProperty, which allows to define a new method directly on the object (more info about that you can find on MDN).

// Examples
// var arr = [0, 1, 2, 2, 3];
// arr.numberOfOccurrences(0) === 1;
// arr.numberOfOccurrences(4) === 0;
// arr.numberOfOccurrences(2) === 2;
// arr.numberOfOccurrences(3) === 1;

Object.defineProperty(Array.prototype, 'numberOfOccurrences',{ 
  value : function numberOfOccurrences(element) {
    return this.filter((el) => el === element).length
  }
 
});

// Different approach 

Array.prototype.numberOfOccurrences = function (element) {
    
    var counter = 0;
    for (var i = 0; i < this.length; i++)
    {
        if (this[i] == element) { counter++; }
    }
    
    return counter;
}