Here is some of the information available to a website running javascript.

A javascript can also access information regarding the size of the screen, bit depth, and the current size of the window. All this information allows for websites to create a unique fingerprint of each user. The more unique these are, the more identifiable a single user is. A browser fingerprint stays persistent even if you delete tracking cookies.

This is why some users choose to block non essential javascript files. A lot of information can be gleaned from a person's browsing habits, such as where they live, their line of work, or their interests.

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