DataView.prototype.setBigInt64()

// Create an ArrayBuffer with a size in bytes
const buffer = new ArrayBuffer(16);
// Highest possible BigInt value that fits in a signed 64-bit integer
const max = 2n ** (64n - 1n) - 1n;
const view = new DataView(buffer);
view.setBigInt64(1, max);
console.log(view.getBigInt64(1));
// Expected output: 9223372036854775807n

dataview.setBigInt64(positionOctet, value [, littleEndian])

var buffer = new ArrayBuffer(8);
var dataview = new DataView(buffer);
dataview.setBigInt64(0, 3n);
dataview.getInt32(0); // 3n