javascript function return true/false with ajax

Question

I am writing the function for checking login like this.

function check(){
  if(login){
    return true
   }else{
    return false
  }
}

if(check()){//do something}

It works if the "login" is some simple checking. Now I want to merge these code in to ajax function which will call api to check the status.

function check(){
    var settings = {
        "async": true,
        "crossDomain": true,
        "url": url,
        "method": "GET",
        "headers": {
            "token": token,
        }
    }
    var aj = $.ajax(settings);

    aj.done(function (response) {
        return true
    });
    aj.fail(function () {
        return false
    })
}

the ajax in work when i just calling , but these function returns "undefined”

How can I fix this?