Modular multiplicative inverse (mod 2^w) in JavaScript

inv.js

/**
 * Modular multiplicative inverse (mod 2^w).
 *
 * Find x, s.t.
 *
 *     ax ≡ 1 (mod 2^w)
 *
 * Based on Hurchalla, J. (2022). “An Improved Integer Modular Multiplicative
 * Inverse (modulo 2^w)”. https://arxiv.org/abs/2204.04342
 *
 * @param {number | bigint} a A positive odd integer to invert.
 * @param {number}          w Bit width.
 */
export const inv = (a, w = 32) => {
    let u = x => BigInt.asUintN(w, x)

    a = u(BigInt(a));

    let x = u(3n * a) ^ 2n;
    let y = u(1n - u(a * x));

    let n = (w >> 2) - 1;

    while (n) {
        x = u(x * u(1n + y));
        y = u(y * y);

        n = n >> 1;
    }

    return x;
};

inv.test.js

if (inv(105n, 8) !== 217n) throw new Error('8-bit test failed.');
if (inv(217n, 8) !== 105n) throw new Error('8-bit test failed.');

if (inv(62169n, 16) !== 28009n) throw new Error('16-bit test failed.');
if (inv(28009n, 16) !== 62169n) throw new Error('16-bit test failed.');

if (inv(277803737n, 32) !== 2897767785n) throw new Error('32-bit test failed.');
if (inv(2897767785n, 32) !== 277803737n) throw new Error('32-bit test failed.');

if (inv(12605985483714917081n, 64) !== 15009553638781119849n) throw new Error('64-bit test failed.');
if (inv(15009553638781119849n, 64) !== 12605985483714917081n) throw new Error('64-bit test failed.');

if (inv(327738287884841127335028083622016905945n, 128) !== 266050134430354249485913702517209329001n) throw new Error('128-bit test failed.');
if (inv(266050134430354249485913702517209329001n, 128) !== 327738287884841127335028083622016905945n) throw new Error('128-bit test failed.');