JavaScript Date prototype Property

Date.prototype.myMet = function() {
    if (this.getMonth() == 0){this.myProp = "January"};
    if (this.getMonth() == 1){this.myProp = "February"};
    if (this.getMonth() == 2){this.myProp = "March"};
    if (this.getMonth() == 3){this.myProp = "April"};
    if (this.getMonth() == 4){this.myProp = "May"};
    if (this.getMonth() == 5){this.myProp = "June"};
    if (this.getMonth() == 6){this.myProp = "July"};
    if (this.getMonth() == 7){this.myProp = "August"};
    if (this.getMonth() == 8){this.myProp = "Spetember"};
    if (this.getMonth() == 9){this.myProp = "October"};
    if (this.getMonth() == 10){this.myProp = "November"};
    if (this.getMonth() == 11){this.myProp = "December"};
};

var d = new Date();
d.myMet();
var monthname = d.myProp;