Hi.
It’s to make sure that the result of window.EventSource is converted to a boolean variable, even if it doesn’t return a boolean type.
 
Regards,
Sara

humm. ok. I’ll take your word on that, and obviously it works.  Strange that I can’t find any language reference that explains that operator, and I’ve never seen anything like it before.  Would you have a link to something that defines it, or parse it out a bit?  Thanks.

Hi.
I was also confused about that. It’s the notation used by the library examples. So, we decided to keep it like that.
https://riptutorial.com/javascript/example/3047/double-negation—-x-
Regards,
Sara

Hi Sara, thank you for the link which nicely explains it.  It also helped me understand negation of numerical variables versus string variables:

x !== 0  // instead of !!x in case x is a number

x != null  // instead of !!x in case x is an object, string or undefined

so assuming window.EventSource is a number (it’s return value), another way to write the
statement would be: if (window.EventSource !== 0). But if it’s not a number,
it would be if (window.EventSource !=0). !! isn’t so bad after all 🙂 Thanks, and this can be closed out.