Object oriented Javascript, why won't an object's methods work from within my init method?

Question

This code is adapted from mozilla's intro to object oriented js page: Introduction to Object-Oriented JavaScript

When I run the following javascript code, I don't get the "hello" alert indicating that sayHello was called correctly. In the mozilla docs, the creation and calling of the person objects does not fall into an init function- which I copied into the bottom example. What gives?

window.onload = init();

function init()
{
    var person1 = new Person('Male');
    var person2 = new Person('Female');

    // call the Person sayHello method.
    person1.sayHello(); // hello
}

function Person(gender) {
  this.gender = gender;
  alert('Person instantiated');
}

Person.prototype.sayHello = function()
{
  alert ('hello');
};

working example:

function Person(gender) {
  this.gender = gender;
  alert('Person instantiated');
}

Person.prototype.sayHello = function()
{
  alert ('hello');
};

var person1 = new Person('Male');
var person2 = new Person('Female');

// call the Person sayHello method.
person1.sayHello(); // hello

lonesomeday · Accepted Answer · 2011-12-15 22:05:06Z

3

window.onload = init();

There's your problem. This runs the init method, then applies the return value (undefined) as the onload property of the window. So nothing happens onload; everything happens immediately. This means that it happens before you modify Person.prototype.

Do this instead to delay execution:

window.onload = init;

answered Dec 15, 2011 at 22:05

lonesomeday

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4 Comments

Dark Falcon Over a year ago

You also need to do that line after defining the init function, not before.

lonesomeday Over a year ago

@DarkFalcon init is a function declaration, so will be hoisted.

lonesomeday Over a year ago

Although I agree, it's best to declare functions at the top of their containing scope.

eric Over a year ago

this works, and it seems to work whether or not that line lives before or after the init declaration.

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