Pure JavaScript Binary Manipulation

Question

As a personal project, I'm writing a 6502 emulator in JavaScript (HTML5 based). I'm porting some bits of it from a predecessor I created in C. To load in the files (ROMs in my case), I could use this C code:

unsigned char* buffer = calloc(1, 4096);
FILE* file = fopen("xyz", "rb");
fread(buffer, 1, 4096, file);
fclose(file);

and access it like this:

char firstChar = buffer[0];
short nextShort = (buffer[2] << 8) | buffer[1];
free(buffer);

Now, I need to be able to port this to JavaScript. For input, I can do something like this using a file input:

var file = document.getElementById("picker").files[0];
var reader = new FileReader();
reader.readAsDataURL(file);

From here, I have reader.result as a giant, base64 encoding of the file. I need a way to access the binary file as I did in the beginning C example, where I can simply get the values (or use simple bitwise operations)

I'm guessing the most feasible solution would be an array of values 0-255, but I just need access, regardless of how.

Bit manipulation is really not my thing, but I believe you should be reading the file into an ArrayBuffer with FIleReader.readAsArrayBuffer. — bfavaretto
– bfavaretto, Commented Jul 14, 2013 at 18:02
Let me suggest you something: add your last update as an answer, and accept your own answer. This can be valuable content for future visitors! — bfavaretto
– bfavaretto, Commented Jul 14, 2013 at 18:18

bobbybee · Accepted Answer · 2013-07-14 18:20:05Z

1

@bfavaretto pointed me in the correct direction

var file = document.getElementById("picker").files[0];
var reader = new FileReader();
reader.readAsArrayBuffer(file);
var buffer = new Uint8Array(reader.result);

Which can then be accessed as buffer[0] for the first byte.

answered Jul 14, 2013 at 18:20

bobbybee

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conceptacid · Accepted Answer · 2015-11-07 21:30:27Z

1

I'm the author of simplebuf library. It allows quick binary parsing/serialization and is purely written in javascript.

Example:

var layout = [
    sb.field("len", sb.type.uint(32)),
    sb.field("padding", sb.type.uint(32)),
    sb.field("id", sb.type.string_dynamic("len"))
];
var original = {len: 4, "padding": 999, "id": "1234"};
sb.write(buffer, 0, original, layout);

https://github.com/conceptacid/simplebuf.js

answered Nov 7, 2015 at 21:30

conceptacid

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1 Comment

Vuwox Feb 21 at 14:59

I would be interested by this, do you still have it somewhere?

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