First, you must escape your base64 string because there are valid base64 characters are that are used in query strings (= for example). You can do this with escape() in javascript.
Next, you'll need to parse the query string, and unescape() the value to get the base64 string again. Parsing the query string is probably out of scope for this question as there are many resources on the web.
To parse the string, first grab the string from location.search and that will return the query string (including ?). Since you don't need the '?' I suggest using .substring(1) to trim it off. Next, you'll want to split() the strings on the ampersand '&' so that you'll get an array like:
['prod_id=MQ==', 'action=add']. Finally, take each string in that array and split it on the first '=', which will give you an Array(2) where the first is the key and the second is the value.
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