Magento 2: Ajax validation in checkout page

So I need to add an AJAX validation for the field VAT number of shipping and billing address in checkout page, which is basically like this:

define([], function () {     return {         validate:function (value){             $  .ajax({                 url: 'path/to/the/validation/controller',                 type: 'get',                 dataType: 'json',                 cache: false             }).done(function (response) {                 return response;             }).fail(function () {                 return false;             });              return false;         }     }; }); 

But as you can see, this validation won’t work because AJAX async is involved, so it will always return false regardless the validation result.

I want to avoid async: false because it causes bad user experience as it freeze the page while validating, so I come up with these work around, but none works:

1. I create a custom error message span below the VAT field, and set the message myself in AJAX callback:

define([], function () {     return {         validate:function (value){             $  .ajax({                 url: 'path/to/the/validation/controller',                 type: 'get',                 dataType: 'json',                 cache: false             }).done(function (response) {                 if (response) {                     $  ('.vat-error-message span').html('');                     window.vatValidation = true; // When submit form, I check this value to accept or rejecting the form submit.                 } else {                     $  ('.vat-error-message span').html($  t('Invalid VAT Number'));                     window.vatValidation = false;                 }             }).fail(function () {                 $  ('.vat-error-message span').html($  t('Invalid VAT Number'));                 window.vatValidation = false;             });              return true;         }     }; }); 

Problem: many other events that validating the form such as saving new shipping or billing address still expecting return result from the validator, which is always true regardless the validation result.

2. I found a validation method named remote where you basically add it into data-validate which is pretty good: (more information here: Magento 2 – jQuery validation remote method)

<input name="vat_id" data-validate="{'remote':'path/to/the/validation/controller'}"/> 

Problem: It only works outside checkout page, e.g edit address in account page. Seem like in checkout page, they use a whole different set of validation rule which isn’t included remote.

My only hope now is trying to get remote works in checkout page by calling it in my custom validation rule like this:

$  .validation.methods['remote'].call(); 

However, it’s just an idea and I haven’t found a way to get it works.