Expression Validator with async functions

Using asynchron functions in expression validator 

                        //Function that returns the value on callback async
function isCountryExist(params) {
    if (params.length < 1) return false;
    var countryName = params[0];
    //If the question is empty then do nothing
    if (!countryName) {
        //It doesn't matter what the function returns. The library is wating for this.returnResult(resultValue) callback
        this.returnResult(false);
        return false;
    }
    var self = this;
    //call the ajax method
    $.ajax({
        url: "https://restcountries.eu/rest/v2/all"
    }).then(function (data) {
        var found = false;
        var countries = data;
        for (var i = 0; i < countries.length; i++) {
            if (countries[i].name == countryName) {
                found = true;
                break;
            }
        }
        // return the value into the library. Library is waiting for this callback
        self.returnResult(found);
    });
    //May return any value. The library will ignore it.
    return false;
}

// It is important to tell the library that the function is async, the last parameter
// and you will return the value on this.returnResult callback
Survey.FunctionFactory.Instance.register("isCountryExist", isCountryExist, true);

/*
If you do not need to support es5 (IE), then you may use async/await operators
async function isCountryExist(params) {
    if (params.length < 1) return false;
    var countryName = params[0];
    if (!countryName) {
        this.returnResult(false);
        return false;
    }
    var self = this;
    var res = await $.ajax({
        url: "https://restcountries.eu/rest/v2/all"
    }).then(function(data) {
        var found = false;
        var countries = data;
        for (var i = 0; i < countries.length; i++) {
        if (countries[i].name == countryName) {
            found = true;
            break;
        }
        }
        self.returnResult(found);
    });
    return false;
}
*/

    
        Survey.StylesManager.applyTheme("default");
    


var json = {
  elements: [
    {
      type: "text",
      name: "country",
      title: "Type a country:",
      validators: [
        {
          type: "expression",
          expression: "isCountryExist({country}) = true",
          text: "Please type the country correctly"
        }
      ]
    }
  ]
};

window.survey = new Survey.Model(json);


    survey.onComplete.add(function(result) {
        document.querySelector('#surveyResult').textContent =
            "Result JSON:\n" + JSON.stringify(result.data, null, 3);
    });


survey.render("surveyElement");


                    
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