Unexpected output with Javascript

Question

Here is a JavaScript code snippet at http://jsfiddle.net/QWXf4/1/

var x = 5;

function PartyWithoutX(person) {
    // This property will go to window and pollute the global object
    dance = function (person) {
        document.write("Hello " + person.getFullName() + ", Welcome to the Party.");
    };

    this.showX = function () {
        // This will change the global x variable
        x = 25;
        document.write("Value of x is " + x);
    };
}

function Person(firstName, lastName) {
    this.firstName = firstName;
    this.lastName = lastName;
}

Person.prototype.getFullName = function () {
    return this.firstName + " " + this.lastName;
};

var dinesh = new Person("Dinesh", "Singh");

var p1 = new PartyWithoutX(dinesh);

document.write(window.dance(dinesh) + "; Enjoy!!");
document.write("<br/>");
document.write(p1.showX());
document.write("<br/>");
document.write(window.x);
document.write("<br/>");

The output as you can check is

Hello Dinesh Singh, Welcome to the Party.undefined; Enjoy!!
Value of x is 25undefined
undefined

I was expecting

Hello Dinesh Singh, Welcome to the Party; Enjoy!!
Value of x is 25
undefined

Why do I get "Party.undefined" and "25undefined" in the output.

Whats going on here?

document.write does not just "call" your own function, it prints the returned result. — Jongware
– Jongware, Commented Oct 23, 2013 at 19:52
you could replace your document.write with console.log() and view the output in your console window. then reveal what you really want by appending the values to a variable, which then gets appended to the DOM. — klewis
– klewis, Commented Oct 23, 2013 at 19:59

Community · Accepted Answer · 2017-05-23 11:57:21Z

2

When you do

document.write(p1.showX());

you're doing

document.write("Value of x is " + x);

and after that doing

document.write(undefined);

because p1.showX() returns undefined.

But as Pointy pointed, you should not use document.write at all, especially after the document was loaded.

edited May 23, 2017 at 11:57

CommunityBot

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answered Oct 23, 2013 at 19:52

Denys Séguret

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Comments

Pointy · Accepted Answer · 2013-10-23 19:52:53Z

1

You're passing the results of those function calls to document.write(). They don't return anything, so you get those "undefined" results.

answered Oct 23, 2013 at 19:52

Pointy

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Comments

user1981472 · Accepted Answer · 2013-10-23 19:53:57Z

1

Replace

document.write(p1.showX());

with

p1.showX();

Because the first prints the results of pi1.showX() which is undefined.

answered Oct 23, 2013 at 19:53

user1981472

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