DataView.prototype.getBigInt64()

// ArrayBuffer 
const buffer = new ArrayBuffer(16);

//  64 
const max = 2n ** (64n - 1n) - 1n;

const view = new DataView(buffer);
view.setBigInt64(1, max);

console.log(view.getBigInt64(1));
// : 9223372036854775807n

getBigInt64(byteOffset)
getBigInt64(byteOffset, littleEndian)

const { buffer } = new Uint8Array([0, 1, 2, 3, 4, 5, 6, 7, 8, 9]);
const dataview = new DataView(buffer);
console.log(dataview.getBigInt64(1)); // 72623859790382856n