Fix memory leak in Element.removeData()
#1077
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Without this change, it is very easy for an application to slowly leak memory as Raphael continues to add new keys to `eldata`. This memory leak turns out to be a problem for [Piwik](https://piwik.org/), which uses kartographer.js (which uses raphael) to draw an SVG image of the United States in its analytics dashboard ([see demo](https://demo.piwik.org/index.php?module=CoreHome&action=index&idSite=3&period=day&date=yesterday)). Every time the dashboard is redrawn, it spawns ~700 new Elements with data. [I've submitted a pull request that appropriately cleans up this data when the user navigates away from the page using `removeData`](matomo-org/matomo#11350), but Raphael leaves an empty object in the `eldata` map keyed on the element's old ID. Since kartographer/Piwik does not reuse IDs, `eldata` will grow continually on each redraw until the application is out of memory.
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Prior to this change,
Element.removeData()replaces an element's data with an empty object. This behavior results in a slow memory leak for applications that do not re-use IDs, as Raphael continues to add new keys and objects toeldataover time.This memory leak turns out to be a problem for Piwik, which uses
kartographer.js (which uses raphael) to draw an SVG image of the United States
in its dashboard (see demo).
Every time the dashboard is redrawn, it spawns ~700 new Elements with data. I've submitted a pull request that appropriately cleans up this data when the user navigates away from the page using
removeData, but Raphael leaves an empty object in theeldatamap keyed on the element's old ID. Since kartographer/Piwik does not reuse IDs,eldatagrows continually on each redraw until the application is out of memory.