Get inner width of an element

HTML

<div id="test">If a scrollbar appears, the width isn't valid</div>

CSS

div {
    background-color: #5555FF;
    color: white;
    width: 100%;
}

JavaScript

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

var test = document.getElementById("test");

// replace % value with pixels
test.style.width = getInnerWidth(test) + "px";