How do I wait for ajax responce before further code execution?

Question

I have next function that return me the AJAX responce or default false value:

function getResults() {
    // get json object from the database
    var json = false // default value
    var data = new FormData()
    var xhr = new XMLHttpRequest()
    xhr.open('POST', "php/get_survey_results.php", true)
    xhr.onload = function(){
        var jsonString = this.responseText
        json = JSON.parse(decodeURIComponent(jsonString))
        return json
    };
    xhr.send(data); 
    return json
}

function foo() {
    var data = getResults() // returns false, because AJAX responce gets later
    // processing data...
}

AJAX responce is coming later on +- 300 milliseconds and that is why getResults returns false. I want it to wait until AJAX responce will be get or wait some seconds before returning false if not result get. How could I do it ?

I need default value becaue if I remove it - next code in foo will be executed but data variable will be empty