Javascript onload and script callback functions, which takes the precedence? -

i'm loading external script uses callback function, returns specific data. if data not received error should displayed.

here code i've made:

<script> //setting initial state function work once var visitors_loaded=false;   var my_callback = function( data ) {     if (visitors_loaded) return 0;     if (data) {          //success: callback function called , has proper data     visitors_loaded=true;     alert(json.stringify(data));     }     else alert ('error'); //something went wrong }; </script> <script onload="my_callback(null)" onerror="my_callback(null)" src="https://api.clicky.com/api/stats/4?site_id=32020&sitekey=9a19b1a4d1171193&type=visitors&date=this-month&output=json&json_callback=my_callback"></script> 

as can see... many things can go wrong script, naturally added onerror event. on error event fires if change host name or domain of script non-existent.

however, if make changes url of script, can still connects server , fires onload event instead. callback function not called invalid requests, added onload handler well.

now problem is, if loaded , data returned, fire both, callback function , onload. have noticed callback function triggered before onload , set visitors_loaded variable handler function called once.

so far works in js fiddle , offline site wonder if expected behavior? json_callback function have precedence before onload handler?

https://jsfiddle.net/5sfk9ht5/4/

will json_callback function have precedence before onload handler?

if external script calls my_callback synchronously yes.

the scripting section in official html5 specification describes how these things supposed work. specification quite general , has deal lot of details conserning encoding, cors, ignore-destructive-writes counter , on. question don't care these specifics.

in step 4 there note:

note: script compiled , executed.

and in step 7 load event fired:

fire simple event named load @ script element.

so specification defines load event fired after script has been executed.

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as see specification tells why onerror event not fired if change url. error event created if loading script fails. requests https://api.clicky.com/api/stats/ return http 200 status. invalid urls return xml , syntaxerror thrown. not cause onerror handler triggered.

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as others have mentioned if callback called asynchronously can call callback after onload event. don't see reason why async in external script.