javascript - how to create new callback function -


in code below i'm adding callback function list in loop.

every item in list call call log >>item30

can please tell me why? there way create new function() call can log 

item0 item1 item2 item3 item4 , on .......

......

  (var j = 0 ; j < 30 ; j += 1) {        addthisthingtolist(function () {           console.log( "item" +j );       });    }

this happens if function addthisthingtolist() using asynchronous behavior (like ajax calls) , callback called time later after for loop has run course , it's index value @ ending value.

you can fix closure freeze loop value separately each call addthisthingtolist() this:

  (var j = 0 ; j < 30 ; j += 1) {       (function(index) {           addthisthingtolist(function () {               console.log( "item" + index);           });       })(j);   }

working demo: http://jsfiddle.net/jfriend00/a5cjg/

by way of explanation, iife (immediately invoked function expression). variable j passed iife , becomes named argument function named index. then, inside function, can refer index value of j frozen uniquely , separately each call addthisthingtolist(). have named argument index j in case have overriden higher scoped j, prefer use separate variable name more clear what.

here's reference on iife concept if want read more it:

http://benalman.com/news/2010/11/immediately-invoked-function-expression/


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