How to make a browser hog your CPU:

while(true) { console.log("true"); }

(You will have to close the page to stop that behavior)

SHA512'ing random numbers is even more effective

Firefox will show a warning "a website is slowing your computer" and will offer you to stop that script

@enen DDoS network in JavaScript

@b3b3 there are technices to use other protocols via javascript ajax-calls. We're talking about protocols that discard invalid content, like irc, some printer protocols and old versions of smtp. It's amazing this even works.
And of course you can use the scan the network for http-servers too.