Javascript width of div, returns 0

Try document.getElementById("mydiv").offsetWidth instead of .style.width

Use this instead:

document.getElementById("areaDraw").offsetWidth

Edit: As requested, an explanation of why this works (just as an extra reference).

It's because the property style.width is the width as defined explicitly in the CSS, wheareas offsetWidth finds the actual width of the element as it's displayed in the browser.