Javascript variables in HTML attributes

I know there are a lot of posts about this, but I can't find an answer to my specific problem.

I would like to make a JS variable the value of an HTML attribute:

<script> var screenWidth = screen.width </script> 

<img src="images/title.png" width="VARIABLE HERE" style="margin-top: 3%"`

"VARIABLE HERE" is where I would want the screenWidth variable to go. What is the best of going about this?

This should work:

<script>var screenWidth = screen.width;</script> 
...
<img src="images/title.png" onload="this.width=screenWidth;" style="margin-top: 3%">

Give the tag an id.

i.e. <img id="xxxx" ...>

Then use

 document.getElementById('xxx').setAttribute('width', 'somevalue')

See setAttribute

Or use JQuery as the other poster noted