StuffInt64BytesIntoNumber.js

function stuffInt64BytesIntoNumber(_arrayBuffer)
{
    // get a DataView over the ArrayBuffer
    var dv = new DataView(_arrayBuffer);

    // read bytes from DataView into array
    var bytes = [];
    for(let i=0; i<8; i++) {
        bytes.push( dv.getUint8(i) );
    }

    // turn each byte into a padded, binary string
    var binaryStringsArr = bytes.map(function(b) { 
        return b.toString(2).padStart(8, "0");
    });

    // concat into binary string
    var binaryString = binaryStringsArr.join('');

    // parse as Number (accurate up to 52 bits, precision loss after that)
    return parseInt(binaryString, 2);
};