How to set up synchronization when executing the ajax function in jquery_jquery

function existProduct(obj){

var productname = escape(obj.val());
$.getJSON("../product/searchProductByName.action",datat,function(data){
var falg;
falg = data[0]['existproduct'];
//falg returns no or yes,
return falg;
});

}

function validateform( ){
//alert(existProduct($("#products_name")));
if(validateNotnull($("#products_name"))){
if(existProduct($("#products_name "))){
$("#productform").submit();
}else{
$("#spanproduct").html("Business name already exists");
}
}else{
$("#spanproduct").html("Business name cannot be empty");
}
}