DataView

const littleEndian = (() => {
  const buffer = new ArrayBuffer(2);
  new DataView(buffer).setInt16(0, 256, true /*  */);
  // Int16Array 
  return new Int16Array(buffer)[0] === 256;
})();
console.log(littleEndian); // true  false

function getUint64(dataview, byteOffset, littleEndian) {
  //  64  32 4 
  const left = dataview.getUint32(byteOffset, littleEndian);
  const right = dataview.getUint32(byteOffset + 4, littleEndian);

  //  32 
  const combined = littleEndian
    ? left + 2 ** 32 * right
    : 2 ** 32 * left + right;

  if (!Number.isSafeInteger(combined))
    console.warn(combined, " MAX_SAFE_INTEGER");

  return combined;
}

const BigInt = window.BigInt,
  bigThirtyTwo = BigInt(32),
  bigZero = BigInt(0);
function getUint64BigInt(dataview, byteOffset, littleEndian) {
  //  64  32 4 
  const left = BigInt(dataview.getUint32(byteOffset | 0, !!littleEndian) >>> 0);
  const right = BigInt(
    dataview.getUint32(((byteOffset | 0) + 4) | 0, !!littleEndian) >>> 0,
  );

  //  32 
  return littleEndian
    ? (right << bigThirtyTwo) | left
    : (left << bigThirtyTwo) | right;
}

const buffer = new ArrayBuffer(16);
const view = new DataView(buffer, 0);

view.setInt16(1, 42);
view.getInt16(1); // 42