javascript - callback event.dataTransfer.setData -

so want drag , drop takes little while process, i'm using callback. .datatransfer.setdata call nothing run callback. datatransfer.setdata works mycallback fine, , know callback running console log.

any ideas or suggestions doing better?

handledragstart = function(event) {   var foo = [];   this.processfoo(foo,mycallback);    mycallback = function() {     console.log('running callback')     var msg = json.stringify(foo);     event.datatransfer.setdata('text/plain', msg); //won't work     }    //event.datatransfer.setdata('text/plain', 'test message'); //works if uncomment }  processfoo = function(foo, callback) {   //pretend has talk external server , takes while respond   foo.push("bar");   callback() } 

thanks!