randomBigInt.js

export default function randomBigInt(max) {
	const segment = Number.MAX_SAFE_INTEGER;
	const bigSegment = BigInt(segment);
	const maxArray = [];
	for (let bigMax = BigInt(max), zero = 0n; bigMax > zero; bigMax /= bigSegment) {
		maxArray.unshift(Number(bigMax % bigSegment));
	}
	const superSum = (a, b) => a * bigSegment + b;
	return function() {
		let unlimited = false;
		return maxArray.map(n => {
			let result;
			if (unlimited) {
				result = Math.floor(Math.random() * segment);
			} else {
				result = Math.floor(Math.random() * n);
				if (result < n) {
					unlimited = true;
				}
			}
			return BigInt(result);
		}).reduce(superSum, 0n);
	};
}