一段代碼看懂原型對象中的callback

代碼圖示

var s = [23, 65, 98, 5];

Array.prototype.myMap = function (callback) {
    var newArray = [];

    this.forEach(value => newArray.push(callback(value)))

    return newArray;
};

var new_s = s.myMap(function (item) {
    return item * 2;
});