JSFiddle - React, Tailwind, and code Playground

JavaScript

var myPopulation = {
    series: [{
        data: function() {
                function retrieve() {
                    var httpRequest = new XMLHttpRequest();
                    var result
                    httpRequest.onreadystatechange = function() {
                        if (httpRequest.readyState === 4) {
                            if (httpRequest.status === 200) { 
                                //Do whatever you want
                                result = httpRequest.responseText.substr(0, 80)
                            }
                        }
                    };
                    
                    //Added 3rd argument to make the call syncronous
                    httpRequest.open('GET', "http://fiddle.jshell.net", false);
                    httpRequest.send();
                    
                    //added return statements
                    return result
                }
            
                //added return statements
                return retrieve();
            }()
    }]};
    
console.log(myPopulation.series[0].data)