永远不要永远不要在 AJAX 调用中使用async: false。
您需要：
一个API Package 器，用于方便和以后的代码可读性。

const API = {
  email_exists: function (email) {
    return $.post('/api/v3/email_exists', {
      email: email
    }).then(function (result) {
      // evaluate result & return true (email exists) or false (email is free)
      return ...;
    }).fail(function (jqXhr, status, error) {
      showFlashMessage('An error occured, please try again later.', 'error');
      console.log('Error in email_exists', email, jqXhr, status, error);
    });
  },
  // add other API functions in the same way
};

和async validation rule，它们调用您的API：

ko.validation.rules.emailFromAPI = {
  async: true,
  validator: function (val, params, callback) {
    API.email_exists(val).done(function (exists) {
      callback(!exists);    // validation is successful when the email does not exist
    });
  },
  message: 'Please enter a valid email.'
};

使用此规则的可观察对象：

email: ko.observable((ref = REGISTRY.unsavedUser) != null ? ref.email : void 0).extend({
  required: true,
  emailFromAPI: true,
  focus:true
}),

由于 AJAX 是异步的，你必须等待服务器响应，返回ajax调用不会等待服务器调用解析。解决这个问题的一种方法是使用回调（另一种方法是使用async/await进行承诺）

var email_exist_check = function(email, onSuccess, onError) {
  $.ajax({
    url: '/api/v3/email_exists',
    method: 'POST',
    async: false,
    data: { email },
    success: function(data){
      console.log(data);
      if (data && !data.error) { return onSuccess(); }
      return onError(data && data.error);
    },
    error: function (response, status) {
       onError(response);
    }
  });
};

// After doing a front-end email verification
email_exist_check(ref.email, () => {
    console.log('Email is valid');
}, (error) => {
    console.error('Something went wrong', error);
});