This is a thin wrapper arround the BigInt class from https://github.com/peterolson/BigInteger.js. It has the same methods, plus convenience methods to convert to and from uint8 arrays. Internally it uses the native BigInt type if supported by the environment.
npm install --save bigint-polyfill
const BigInt = require('bigint-polyfill');
const a = new BigInt(2n ** 63n - 1n); // in Node.js 10.4+ and supported browsers
// const a = new BigInt('9223372036854775807'); // in other environments
const b = a.divide(2n);
b.toUint8Array();
> Uint8Array [ 63, 255, 255, 255, 255, 255, 255, 255 ]
const b = BigInt.fromUint8Array(new Uint8Array([ 63, 255, 255, 255, 255, 255, 255, 255 ]));
b.toString()
> 4611686018427387903
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