Difference between readAsBinaryString and readAsText using FileReader
So as an example, when I read the π character (\u03C0) from a File using the FileReader API, I get the pi character back to me when I read it using FileReader.readAsText(blob) which is expected. But when I use FileReader.readAsBinaryString(blob), I get the result \xcf\x80 instead, which doesn't seem to have any visible correlation with the pi character. What's going on? (This probably has something to do with the way UTF-8/16 is encoded...)
Answer
FileReader.readAsText takes the encoding of the file into account. In particular, since you have the file encoded in UTF-8, there may be multiple bytes per character. Reading it as text, the UTF-8 is read as it is, and you get your string.
FileReader.readAsBinaryString, on the other hand, does exactly what it says. It reads the file byte by byte. It doesn't recognise multi-byte characters, which in particular is good news for binary files (basically anything except a text file). Since π is a two-byte character, you get the two individual bytes that make it up in your string.
This difference can be seen in many places. In particular when encoding is lost and you see characters like é displayed as é.