Async or Sync bcrypt function to use in node.js in order to generate hashes?

I'm currently trying to make authentication module for my project in node.js?

I've already seen some examples of using bcrypt to generate hashes, i.e.

https://github.com/bnoguchi/mongoose-auth/blob/master/lib/modules/password/plugin.js https://github.com/Turbo87/locomotive-passport-boilerplate/blob/master/app/models/account.js

However, for some reason they are using bcrypt.hashSync() function. Since bcrypt is good because it's time-consuming, wouldn't it be wiser to use asynchronous function instead in order to not block the code, i.e:

User.virtual('password')
.get( function () {
    return this.hash;
})
.set( function (password) {
    bcrypt.hash('password', 10, function(err, hash) {
        this.hash = hash;
    });
});

Could you please explain me which way is better and why? Thank you!