getAttribute of elements with some class doesn't work

Question

I have some input fields with a specific class. They all have a data attribute, say data-somedata with an integer value. I have to calculate the sum of these data attributes. I tried to use getAttribute in combination with getElementsByClassName, but that doesn't work.

I'm using this code:

<input class="datainput" value="The 1st input" type="text" data-somedata="8"/>
<input class="datainput" value="The 2nd input" type="text" data-somedata="15"/>
<div id="result"></div>

var fields = document.getElementsByClassName('datainput');
var result = 0;
for (var i in fields) {
    result += fields[i].getAttribute('data-somedata');
}
document.getElementById('result').innerHTML = 'The sum is: ' + result;

See it live on jsFiddle

In the (firefox) console, I get:

TypeError: fields[i].getAttribute is not a function

Why is this? And how can I fix it?

I do not want to use jQuery unless absolutely necessary (I don't suppose it is).

Answer 1

Instead of using the for-in statement, use a simple for statement as in:

var fields = document.getElementsByClassName('datainput');
var result = 0;
for (var i = 0; i < fields.length; i ++) {
    var val = parseInt(fields[i].getAttribute('data-somedata'), 10);
    result += val;
}
document.getElementById('result').innerHTML = 'The sum is: ' + result;

Also, the result of getAttribute is a string, you must cast it into an integer using parseInt() before performing an add operation. Otherwise a string concatenation will be performed making your result:

The sum is: 0815

Updated Fiddle

The reason why the for-in statement didn't work is for-in iterates over properties of the target object, which in your case is HTMLCollection(result of getElementsByClassName). In fact, you can make it work with a for-in using this ugly snippet:

var c = 0;

for(var i in fields) {
    if(c == fields.length) break;
    c ++;    
    console.log(fields[i].getAttribute('data-somedata'));
}

The first n = fields.length properties of HTMLCollection are its items' indices.

Answer 2

Check this:

var fields = document.getElementsByClassName('datainput');
var result = 0;

for (var i = 0; i < fields.length; i++) {        
    result += parseInt(fields[i].getAttribute('data-somedata'));
}
document.getElementById('result').innerHTML = 'The sum is: ' + result;

On jsfiddle

Answer 3

As far as the reason, the for-in loop will cycle over all properties of the object, including functions (and inherited properties).