Migrate from argon2

Let's users be your collection of registered users. Each of those users should have the hash computed with argon2 stored somewhere. We will assume that this was stored in argon2 property of each user.

users.forEach(user => {
  var hash = JSON.stringify({
    hash: user.argon2,
    func: 'argon2'
  });
  user.hash = hash;  // Now you can pass user.hash to the verify function of
                     // this package.
});