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Editorial Team
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Asked: 2026-05-24T16:31:38+00:00

I’m trying to use XMLHttpRequest to download binary data. When I set the charset

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I’m trying to use XMLHttpRequest to download binary data. When I set the charset to ‘x-user-defined’ so the browser won’t mess with the data, the browser does not set each byte of the response to the low byte of a UTF-16 string; instead, some bytes are prefixed by 0xf7. I am specifically trying to do this in an Android WebView, but I believe Firefox does the same thing.

Why is this happening? I’m getting exactly the data I want, but I have to && 0xff against each byte to strip away the unwanted 0xf7.

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1 Answer

  1. Editorial Team

    This solution is mentioned in Mozilla docs, and the explanation comes from Marcus Granado (text no longer available, but read the backup from web.archive.org):

    The charset x-user-defined uses the UNICODE Private Area
    0xF700–0xF7ff to map its range.

    When you do & 0xff you simply discard the byte in the high-order position.

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