Javascript optional if else inside constructor

Question

Here's the code to start with.

function person(name, age, child){
    this.name = name;
    this.age = age;
    if(this.child == undefined){
        this.child = 'default';
    }else{
        this.child = child;
    }
}
var sarah = new person('sarah',34,true);

document.write(sarah.child+' ');

So I'm trying to make an optional property inside a constructor function. But whatever I put inside the child parameter it always says 'default' when printed out. I am exceedingly new to JS, just came off php. No idea why this isn't working. I've looked at other questions, tried to follow, but what I try from them doesn't seem to help.

this.child is always undefined, since you did not define it (you only defined this.name and this.age). Btw, you should use === to compare with undefined. — zerkms
– zerkms, Commented Apr 1, 2015 at 20:59
you just need to check child: if (child == undefined) {. — dfsq
– dfsq, Commented Apr 1, 2015 at 21:00
Oh damn. I took that initial definition out thinking the problem I had before was me defining it twice. Thank-you! And thankyou for the === tip. — Russ
– Russ, Commented Apr 1, 2015 at 21:01
@RussellKitchen: child is the parameter (variable). this.child is a property. — Bergi
– Bergi, Commented Apr 1, 2015 at 21:02
No, this.child is a property of the instance, which is always undefined before you define it. And you are only defining it later inside if check. — dfsq
– dfsq, Commented Apr 1, 2015 at 21:03

Douwe Pieter Reitsma · Accepted Answer · 2015-04-01 21:08:28Z

2

Why don't you just use child = child || 'default' instead of an if else statement?

This achieves the same thing.

answered Apr 1, 2015 at 21:08

Douwe Pieter Reitsma

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Comments

Filipe · Accepted Answer · 2015-04-01 21:16:45Z

0

The correct code is below:

function person(name, age, child){
    this.name = name;
    this.age = age;
    if(child == undefined){
        this.child = 'default';
    }else{
        this.child = child;
    }
}
var sarah = new person('sarah',34,true);

document.write(sarah.child+' '); // true

The explanation is that you are always comparing this.child with undefined but what you want is test the argument child instead of this.child.

A shortcut can be provided, instead of use if/else:

this.child = child || 'default';

answered Apr 1, 2015 at 21:16

Filipe

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