let localEndianness = () => {
    let uInt32 = new Uint32Array([0x12345678]); // have a 4 bytes long thing
    let uInt8 = new Uint8Array(uInt32.buffer); // split it in 1 byte long things
     // Now, uInt8[0] returns the first byte of uInt32 according to the machine running this program. 
    if(uInt8[0] === 0x78) { // here we got the byte of lesser order
        return 'little';
    } else if (uInt8[0] === 0x12) { // here we got the byte of bigger order
        return 'big'; 
    } else { // you may check for older or stranger behaviours here, and even endianness on bits rather than bytes if needed. But hey, ECMAScript languages usually don't run on such rare machines.  
        return 'mixed'; // mixed yes, but how mixed ?
    }
};