Session variables without cookies. Brilliant but terrifying hack—you can store up to 2 MB of data in window.name and it persists between multiple pages, even across domains. Doesn’t work with new tabs though, and storing JSON in it and eval()ing it is a bad idea—a malicious site could populate it before sending the user to you.
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The other way to handle larger session data storage would be to store a randomly generated key in the session, and then use that to access server side stored data.
I used something similar where i needed to have the effect of storing files in the session until a process was complete, so i stored them server side but accessed through a client side key when they were needed.