How to copy a value comes in alert in javascript

I have a javascript function with a parameter.
function fn(url)
{
alert(url);
}
This function called when i hit a button.
After i hit the button, the  url value should be copy. and then if i press ctrl+v it should paste on the respective place.
How can i do that?
because now i am showing the url in alert msg show i can't copy that manually... how can i copy that url after hit a button.?
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