How to override onLoad?

I'm using this page that has a very annoying alert(its typical JavaScript alert, 开发者_C百科so any other hacks are welcome) that is displayed onLoad, can I stop it somehow with GM?

P.S. From what I understand GM is executed after the page has already loaded. Is there any way to click "ok" on the alert or override onLoad?

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If the webpage is using the old fashioned method of detecting onLoad, you probably can just do:

unsafeWindow.onload = function() {};

If they are using a JavaScript library to listen for the onLoad or contentReady events I'm not sure how you could override those.