Displaying an 'undefined' message in a JavaScript alert box

Question

Displaying an 'undefined' message in a JavaScript alert box

Hello!

Just made the switch from C#/C++ to JavaScript recently and I'm really enjoying it.

I've encountered a behavior that has me scratching my head, can anyone help explain?

So here's what's happening: when I run this script, I see the expected alert displaying '5.5', but then after closing that alert, another one pops up showing only "undefined". Any ideas why this is happening?

Code snippet:

var myObj = {
    age : 5,
    weight : 5.5,

    toString : function(){
        alert(this.weight);
    }
}

alert(myObj.toString());

Appreciate any insights you could offer.

javascript

Answer 1

Answer №1

Your script triggers the alert() function twice.

The first alert shows the value of this.weight. However, the second alert displays the result of calling the myObj.toString() method. Since you haven't specified a return value for this method, it defaults to undefined.

In most cases, a .toString() method should return a string. To fix this issue, you can update your method like this:

toString : function(){
    return this.weight.toString();
}

With this adjustment, only one alert will appear as demonstrated here: http://jsfiddle.net/eph7x/

After making that change, you can simply use:

alert(myObj);

...because the custom .toString() method will be automatically invoked.

Answer 2

Your script triggers the alert() function twice.

The first alert shows the value of this.weight. However, the second alert displays the result of calling the myObj.toString() method. Since you haven't specified a return value for this method, it defaults to undefined.

In most cases, a .toString() method should return a string. To fix this issue, you can update your method like this:

toString : function(){
    return this.weight.toString();
}

With this adjustment, only one alert will appear as demonstrated here: http://jsfiddle.net/eph7x/

After making that change, you can simply use:

alert(myObj);

...because the custom .toString() method will be automatically invoked.

Displaying an 'undefined' message in a JavaScript alert box

Answer №1

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