How get element width by javascript

Question

what's the JavaScript code for the find the 's width?

(I want get width .FixedBox whit javascript no jquery, but my code don't work.)

alert(document.getElementsByClassName('FixedBox').offsetWidth);

<div class="FixedBox">
    The HTMLElement.offsetWidth read-only property returns the layout width of an element. Typically, an element's offsetWidth is a measurement which includes the element borders, the element horizontal padding, the element vertical scrollbar (if present, if rendered) and the element CSS width.
</div>

Answer 1

The Document.getElementsByClassName() returns NodeList you need to get element from collection to get offsetWidth property. Also put them inside window load callback to execute only after elements are loaded.

window.onload = function() {
  var ele = document.getElementsByClassName('FixedBox');
  alert(ele.length ? ele[0].offsetWidth : 'no elements with the class');
}

<div class="FixedBox">
  The HTMLElement.offsetWidth read-only property returns the layout width of an element. Typically, an element's offsetWidth is a measurement which includes the element borders, the element horizontal padding, the element vertical scrollbar (if present,
  if rendered) and the element CSS width.
</div>

Answer 2

Try This

document.getElementsByClassName('FixedBox')[0].offsetHeight

Answer 3

This might work

$('.FixedBox').load(function () {
    alert(document.getElementsByClassName('FixedBox')[0].getBoundingClientRect().width)
});

alert(document.getElementsByClassName('FixedBox')[0].getBoundingClientRect().width)

<div class="FixedBox">
  The HTMLElement.offsetWidth read-only property returns the layout width of an element. Typically, an element's offsetWidth is a measurement which includes the element borders, the element horizontal padding, the element vertical scrollbar (if present,
  if rendered) and the element CSS width.
</div>