JS: Modular Inverse via the Extended Euclidean Algorithm

// Ref: https://stackoverflow.com/q/29569927
const modInverse = (num, mod) => {
  let a = BigInt(num)
  let m = BigInt(mod)

  let q = [BigInt(0), BigInt(1)]
  for (let i = 2; m % a > 0; i++) {
    const quotient = m / a
    const remainder = m % a

    q[i] = q[i - 2] - (q[i - 1] * quotient)

    m = a
    a = remainder
  }

  let inverse = q[q.length - 1]
  if (inverse < 0) {
    inverse += BigInt(mod)
  }

  return inverse
}