how to use alt of image in javascript t wirte something

i have to use the alt of image how can i set this property in javascript
i m trying this but alt is not coming.............
var img1 = new Image();
img1.src = "images/mall1.jpg";
document.$Image1 = img1;
img1.alt="#cap1";
<div id="cap1" style="display:none">
<a>Royal Meenakshi Mall, Old Madras Road Bangalore</a>.
</div>