使用构造函数创建新对象并在javascript中调用函数

Question

I'm having a little trouble with this code:

window.onload = function() {
// Make a new method isLeapYear and accept the date argument.
Date.prototype.isLeapYear = function(date) {
    if(date.getFullYear() % 4 != 0) {
        return false;
    } else if(date.getFullYear() % 100 != 0) {
        return true;
    } else if(date.getFullYear() % 400 != 0) {
        return false;
    } else {
        return true;
    }
}


// Make a new method daysInMonth and accept the date argument.
Date.prototype.daysInMonth = function(date) {
    if(date.getMonth() === 0 || date.getMonth() === 2 || date.getMonth() === 4 || date.getMonth() === 6 || date.getMonth() === 7 || date.getMonth() === 9 || date.getMonth() === 11) {
        return "31";
    } else if(date.getMonth() === 3 || date.getMonth() === 5 || date.getMonth() === 8 || date.getMonth() === 10) {
        return "30";
    } else if(date.getMonth() === 1) {
        if(date.isLeapYear(date) === true) {
            return "29";
        } else {
            return "28";
        }
    }
}

// Make a new method ageInYears and accept the date argument.
Date.prototype.ageInYears = function(date) {
    return Math.floor((new Date() - date) / 1000 / 60 / 60 / 24 / 365.2595);
}

function Person(firstname, lastname, mi, birthdate) {
    this.firstname = firstname;
    this.lastname = lastname;
    this.mi = mi;
    this.birthdate = birthdate;
    this.age = this.birthdate.ageInYears(this.birthdate);
}

Person.prototype.setBirthdate = function(month,day,year) {
        currentYear = new Date().getFullYear();
        currentMonth = new Date().getMonth();
        currentDay = new Date().getDate();

        if(month > currentMonth && day > currentDay && year > currentYear || month > currentMonth && year > currentYear) {
            return "Month/Day/Year not valid.";
        } else {
            return (new Date(year, month, day)); 
        }
}

var newGuy = new Person("New", "Guy", "J",  Person.setBirthdate(5,7,1989));
}

I get the error "Uncaught TypeError: undefined is not a function person.js:59" It is saying that undefined is not a function on the new Person object. How come? I've been stumped a while, can anyone help me out?

Answer 1

Your setBirthdate method is rather static, and does not modify the instance it is called on. It should not be placed on the prototype, and be named differently.

function Person(firstname, lastname, mi, birthdate) {
    this.firstname = firstname;
    this.lastname = lastname;
    this.mi = mi;
    this.setBirthdate(birthdate); // call prototype methods on the instance
}
Person.prototype.setBirthdate = function(birthdate) {
    this.birthdate = birthdate;
    this.age = this.birthdate.ageInYears(this.birthdate);
};

// place the static validation function on the constructor itself:
Person.makeBirthdate = function(month,day,year) {
    var currentYear = new Date().getFullYear(),
        currentMonth = new Date().getMonth(),
        currentDay = new Date().getDate();

    if (month > currentMonth && day > currentDay && year > currentYear || month > currentMonth && year > currentYear) {
        throw "Month/Day/Year not valid.";
    } else {
        return new Date(year, month, day); 
    }
}
// where you can call it before having instantiated the `newGuy`
var newGuy = new Person("New", "Guy", "J",  Person.makeBirthdate(5,7,1989));