jsbi-dataview.js

import JSBI from 'jsbi';
const useNativeBigIntsIfAvailable = true;
if (useNativeBigIntsIfAvailable) JSBI.useNativeBigIntsIfAvailable();
const BigInt = JSBI.BigInt

DataView.prototype._setBigUint64 = DataView.prototype.setBigUint64;
DataView.prototype.setBigUint64 = function(byteOffset, value, littleEndian) {
    if (typeof value === 'bigint' && typeof this._setBigUint64 !== 'undefined') {
        // the original native implementation for bigint
        this._setBigUint64(byteOffset, value, littleEndian);
    } else if (value.constructor === JSBI && typeof value.sign === 'bigint' && typeof this._setBigUint64 !== 'undefined') {
        // JSBI wrapping a native bigint
        this._setBigUint64(byteOffset, value.sign, littleEndian);
    } else if (value.constructor === JSBI) {
        // JSBI polyfill implementation
        let lowWord = value[0], highWord = 0;
        if (value.length >= 2) {
            highWord = value[1];
        }
        this.setUint32(littleEndian ? 0 : 4, lowWord, littleEndian);
        this.setUint32(littleEndian ? 4 : 0, highWord, littleEndian);
    } else {
        throw TypeError('Value needs to be BigInt ot JSBI');
    }
}

DataView.prototype._getBigUint64 = DataView.prototype.getBigUint64;
DataView.prototype.getBigUint64 = function(byteOffset, littleEndian) {
    if (typeof this._setBigUint64 !== 'undefined' && useNativeBigIntsIfAvailable) {
        return BigInt(this._getBigUint64(byteOffset, littleEndian));
    } else {
        let lowWord = 0, highWord = 0;
        lowWord = this.getUint32(littleEndian ? 0 : 4, littleEndian);
        highWord = this.getUint32(littleEndian ? 4 : 0, littleEndian);
        const result = new JSBI(2, false);
        result.__setDigit(0, lowWord);
        result.__setDigit(1, highWord);
        return result;
    }
}

const number = BigInt(String(2n ** 63n - 1n)); // '9223372036854775807'

console.log(number, String(number));

function toUint8Array(bigint, littleEndian = false) {
    const arr = new ArrayBuffer(8);
    const view = new DataView(arr);
    view.setBigUint64(0, bigint, littleEndian);
    return new Uint8Array(arr);
}

function fromUint8Array(uint8arr, littleEndian = false) {
    const view = new DataView(uint8arr.buffer);
    return view.getBigUint64(0, littleEndian);
}

const arr = toUint8Array(number, false);
console.log(arr);
console.log(fromUint8Array(arr, false), String(fromUint8Array(arr, false)));

const arr2 = toUint8Array(number, true);
console.log(arr2);
console.log(fromUint8Array(arr2, true), String(fromUint8Array(arr2, true)));

/*
Output for polyfilled implementation:

JSBI [ -1, 2147483647, sign: false ] '9223372036854775807'
Uint8Array [ 127, 255, 255, 255, 255, 255, 255, 255 ]
JSBI [ -1, 2147483647, sign: false ] '9223372036854775807'
Uint8Array [ 255, 255, 255, 255, 255, 255, 255, 127 ]
JSBI [ -1, 2147483647, sign: false ] '9223372036854775807'

Output for native implementation:

JSBI [ sign: 9223372036854775807n ] '9223372036854775807'
Uint8Array [ 127, 255, 255, 255, 255, 255, 255, 255 ]
JSBI [ sign: 9223372036854775807n ] '9223372036854775807'
Uint8Array [ 255, 255, 255, 255, 255, 255, 255, 127 ]
JSBI [ sign: 9223372036854775807n ] '9223372036854775807'
*/

Hi @glr0221, thanks for your comment! I see the original issue is still open. I tried to make this into a PR which didn't go anywhere, maybe you could try to contribute there.