The last reference even recommends using event.target:
The FileReader instance is available inside of the event handler via event.target and itís recommended to use that instead of referencing the reader variable directly.
But why is it recommended and widely used? What's wrong with using reader or simpler this instead: DEMO [jsfiddle.net].
I know this and event.target can be different, but in this case they both refer to the FileReader object and result is in fact a FileReader property [developer.mozilla.org] -- just like onload, which is a FileReader event [developer.mozilla.org].