If I get Chrome to show me document.cookie by going into the console and typing document.cookie; it'll give me, say:
"name=John; gender=male";
But then if I type in, say, document.cookie = 5; all it does is add 5; to the start of the string, so I get:
"5; name=John; gender=male";
If I try document.cookie = null; then it doesn't even do anything.
How can this be? It's a variable, isn't it? So why isn't the assignment operator working the way it should? Is it actually just a bit of syntactic sugar rather than a real variable? And if so, what precisely is the sugar covering up?
document.cookie has very special behavior. As you've seen, assigning to it adds (or updates) a cookie (or multiple cookies), rather than replacing all of the cookies. It's very unusual.
Read all about it on MDN.
The string on the right side of the assignment operator to document.cookies should be a semicolon separated list of key-value pairs, i.e. document.cookie = "aKey=5" will set/update the aKey cookie.
So yes, document.cookie shows special behavior.
Here is an example of your "issue". Also, it says the following:
You can delete a cookie by simply updating its expiration time to zero.
来源：https://stackoverflow.com/questions/6791944/how-exactly-does-document-cookie-work