javascript - Difference between readAsBinaryString and readAsText using FileReader - Stack Overflow

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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...)

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...)

• javascript
• string
• html
• utf
• typed-arrays

Share Improve this question Follow asked Feb 19, 2012 at 2:11 gengkevgengkev 1,95922 gold badges2121 silver badges3131 bronze badges 2

• 2 CF80 is the UTF-8 encoding for π. – deceze ♦ Commented Feb 19, 2012 at 2:15
• ...that was probably too obvious, but you can post that as an answer if you like. – gengkev Commented Feb 19, 2012 at 4:37

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2 Answers 2

Sorted by: Reset to default 22

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.

Oh well, if that's all you needed... :)

CF80 is the UTF-8 encoding for π.

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