Question

142

Reading binary data in node.js

rated 0 times [ 146] [ 4] / answers: 1 / hits: 66451 / 7 Years ago, wed, september 27, 2017, 12:00:00

I am writing a JavaScript program that I am executing in the Node.js runtime environment.

I want to read binary data from a file into a buffer, but I keep running into issues.

Here is what I have done so far:

    $ cat test.js 

    > var fs = require('fs'),

    > binary = fs.readFileSync('./binary', 'binary').toString('binary');

    > process.stdout.write(binary.substring(0, 48));

    $ xxd binary

    00000000: 7f45 4c46 0201 0100 0000 0000 0000 0000  .ELF............

    00000010: 0300 3e00 0100 0000 0008 0000 0000 0000  ..>.............

    00000020: 4000 0000 0000 0000 10a0 0000 0000 0000  @...............

    $ node test.js | xxd

    00000000: 7f45 4c46 0201 0100 0000 0000 0000 0000  .ELF............

    00000010: 0300 3e00 0100 0000 0008 0000 0000 0000  ..>.............

    00000020: 4000 0000 0000 0000 10c2 a000 0000 0000  @...............

    00000030: 00                                       .

Notice how a 0xc2 byte is inserted at index 0x29 when reading with node. Why is that? I've stated binary encoding both to readFileSync and toString.
I've also tried ascii but then I get a different and equally wrong result.

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coleman

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3 Years ago

answered 7 Years ago ellisw · Accepted Answer

The 'binary' encoding is an alias for 'latin1', which you clearly don't want when reading non-character data.

If you want the raw data, don't specify an encoding at all (or supply null)*. You'll get a Buffer instead of a string, which you'd then want to use directly rather than using toString on it.

* (Some APIs [like fs.watch] also accept 'buffer', but it's not on the list of encodings and readFileSync doesn't say it does. [Thanks Patrick for providing the list link.])