Two ways
1: Change ajax to synchronization, and you can directly obtain the correct return value.
2: Add your processing logic directly to the ajax callback

Add a function parameter callback to getA, and then use callback(set.a) in the ajax return value.
zAccess set.a like this when using getA: getA(function(a){console.log(a)})

Examples are as follows:

    var a =1;

    function setA(callback){
        $.get('http://localhost/','a=2',function(ret){
            callback(ret.a);
        });
    }
    
    setA(function(a) {
        console.log('a:' + a)
    })

var a;

$.get('http://localhost/','a=2',function(ret){
    setA(ret.a); });
function setA(value){
    a=value;
}

function getA(){
    return a;
    }

Use a temporary variable storage in setA. The AJAX in setA uses synchronous request. After success, the value is stored in the temporary variable, and then the temporary variable is returned by setA

setA() {
    var temp = '';

    $.get({
        url: 'http://localhost/',
        data: {a: 2},
        async: false,
        success: function(ret) {
            temp = ret.a;
            return ;
        }
    });
    
    return temp;
}