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var evt = document.createEvent("HTMLEvents");
evt.initEvent("click", true, true);
var mylink = document.getElementById("watch-video-details-toggle-less").getElementsByTagName("a")[0]; 
mylink.dispatchEvent(evt);

Thank you very much!!

This is for new Youtube:

var evt = document.createEvent("HTMLEvents");

evt.initEvent("click", true, true);

var mylink = document.getElementById("watch-description").getElementsByTagName("button")[0]; 

mylink.dispatchEvent(evt);

Thankyou SamSpade for the fix for new youtube layout : )