Point of information: the ;void(0); isn’t necessary here because the code ends in a for loop, which produces undefined.

(For anyone who doesn’t know about the meaning of void in JavaScript: it’s a unary operator that ignores its operand and produces undefined. What is commonly spelled `void(0)` is briefer as `void 0`. Why don’t people just write `undefined`? For some, because it’s shorter, but mostly for historical reasons: `undefined` is a global property rather than a keyword, and until ES5 (a decade ago), that global property was writeable, so you couldn’t rely on its value because people might do crazy things. And if you’re wondering why a javascript: URL needs to produce exactly the value undefined: if it doesn’t, the browser will convert the value to a string and replace the page with that, loaded as HTML, as demonstrated by this (paste it in your address bar, you can’t click on data: URLs directly for security reasons):

  data:text/html;charset=utf-8,<a href="javascript:'<h1>Here is a page.'">α</a> <a href="javascript:({toString(){return '<h1>See, stringification!'}})">β</a> <a href="javascript:/*even*/null/*gets stringified*/">γ</a> <a href="javascript:undefined">δ</a>

.)

It wasn't actually working for me; it was returning 0 actually. But yeah, this is a good point as well. :D

Huh, turns out my understanding of how it worked was wrong: in things like for loops, it takes the last value encountered. I should have anticipated that, really, given that “last expression” isn’t generally a thing in JavaScript anyway (it’s statement-oriented, not expression-oriented), so javascript: URIs (and I guess on* event handler attributes too, I know they’re a bit weird too) were clearly doing something clever and not-usual-JavaScript. I should delve into specs until I can find it; it seems like it’s at least mildly expression-oriented in character.

I retract the first paragraph of my earlier comment with an apology.

I just put onion2k's version as a script in violent monkey, set to run only on NH.