Get width in pixels from element with style set with %?

I have this element:

<div style="width: 100%; height: 10px;"></div>

I want to get it's width in pixels. I just tried this:

document.getElementById('banner-contenedor').style.width

Which returns 100%. Is it possible to actually get the element's width in pixels with JavaScript?

document.getElementById('banner-contenedor').clientWidth

You want to get the computed width. Try: .offsetWidth

(I.e: this.offsetWidth='50px' or var w=this.offsetWidth)

You might also like this answer on SO.

Not a single answer does what was asked in vanilla JS, and I want a vanilla answer so I made it myself.

clientWidth includes padding and offsetWidth includes everything else (jsfiddle link). What you want is to get the computed style (jsfiddle link).

function getInnerWidth(elem) {
    return parseFloat(window.getComputedStyle(elem).width);
}

EDIT: getComputedStyle is non-standard, and can return values in units other than pixels. Some browsers also return a value which takes the scrollbar into account if the element has one (which in turn gives a different value than the width set in CSS). If the element has a scrollbar, you would have to manually calculate the width by removing the margins and paddings from the offsetWidth.

function getInnerWidth(elem) {
    var style = window.getComputedStyle(elem);
    return elem.offsetWidth - parseFloat(style.paddingLeft) - parseFloat(style.paddingRight) - parseFloat(style.borderLeft) - parseFloat(style.borderRight) - parseFloat(style.marginLeft) - parseFloat(style.marginRight);
}

With all that said, this is probably not an answer I would recommend following with my current experience, and I would resort to using methods that don't rely on JavaScript as much.

You can use offsetWidth. Refer to this post and question for more.

console.log("width:" + document.getElementsByTagName("div")[0].offsetWidth + "px");

div {border: 1px solid #F00;}

<div style="width: 100%; height: 10px;"></div>

Try jQuery:

$("#banner-contenedor").width();

This jQuery worked for me:

$("#banner-contenedor").css('width');

This will get you the computed width

http://api.jquery.com/css/

yourItem.style['cssProperty']

this way you can call the property string dynamically