JavaScript Getting binary representation of an image file


Example

This example is inspired by this question.

We'll assume you know how to load a file using the File API.

// preliminary code to handle getting local file and finally printing to console
// the results of our function ArrayBufferToBinary().
var file = // get handle to local file.
var reader = new FileReader();
reader.onload = function(event) {
    var data = event.target.result;
    console.log(ArrayBufferToBinary(data));
};
reader.readAsArrayBuffer(file); //gets an ArrayBuffer of the file

Now we perform the actual conversion of the file data into 1's and 0's using a DataView:

function ArrayBufferToBinary(buffer) {
   // Convert an array buffer to a string bit-representation: 0 1 1 0 0 0...
   var dataView = new DataView(buffer);
   var response = "", offset = (8/8); 
   for(var i = 0; i < dataView.byteLength; i += offset) {
       response += dataView.getInt8(i).toString(2); 
   }
   return response;
}

DataViews let you read/write numeric data; getInt8 converts the data from the byte position - here 0, the value passed in - in the ArrayBuffer to signed 8-bit integer representation, and toString(2) converts the 8-bit integer to binary representation format (i.e. a string of 1's and 0's).

Files are saved as bytes. The 'magic' offset value is obtained by noting we are taking files stored as bytes i.e. as 8-bit integers and reading it in 8-bit integer representation. If we were trying to read our byte-saved (i.e. 8 bits) files to 32-bit integers, we would note that 32/8 = 4 is the number of byte spaces, which is our byte offset value.


For this task, DataViews are overkill. They are typically used in cases where endianness or heterogeneity of data are encountered (e.g. in reading PDF files, which have headers encoded in different bases and we would like to meaningfully extract that value). Because we just want a textual representation, we do not care about heterogeneity as there is never a need to

A much better - and shorter - solution can be found using an UInt8Array typed array, which treats the entire ArrayBuffer as composed of unsigned 8-bit integers:

function ArrayBufferToBinary(buffer) {
    var uint8 = new Uint8Array(buffer);
    return uint8.reduce((binary, uint8) => binary + uint8.toString(2), "");
}