javascript - Scoping of params in callbacks with closure -

i know there have been many questions asked on topic interested in difference between 2 particular scenarios. here is

scenario 1

   //global scope var a=1, b =2;  var callbackfn = function(param3,param4){     alert(a);             //1  -->defined     alert(b);             //2  -->defined     alert(param1);        //3  -->?      alert(param2);        //4  -->?      alert(param3);        //5  -->defined      alert(param4);        //6  -->defined     alert(localvariable); //7  -->? }   var classobj = {      somememberfunction: function(param1, param2{         var localvariable;          return $.asynccallfn(param1, param2).then(function(){            callbackfn(3,4);         });     }  };  classobj.somememberfunction(5,6); 

scenario 2

    //global scope var a=1, b =2;  var classobj = {      somememberfunction: function(param1, param2){         var localvariable;          $.asynccallfn(param1, param2).then(function(param3,param4){             alert(a);             //1  -->?             alert(b);             //2  -->?             alert(param1);        //3  -->?              alert(param2);        //4  -->?              alert(param3);        //5  -->?              alert(param4);        //6  -->?              alert(localvariable); //7  -->?         });     };  }  classobj.somememberfunction(5,6); 

what outputs of alerts #1 through #6 , more importantly there difference in scenario 1 , scenario 2.

from understanding in scenario 1 callbackfn know nothing param1, param2 , localvariable because not created inside somememberfunction. scenario 2, should somememberfunction not aware of param1, param2 , localvariable well, because closure inside somememberfunction?

thanks

javascript has lexical scope. therefore function (closure) has access variables visible in scope defined in.

example 1

// global scope // foo visible in global scope var foo = 42;  // bar defined in global scope, hence can access foo function bar() {    console.log(foo); } 

example 2

// global scope // foo visible in global scope var foo = 42;  // bar defined in global scope, hence can access foo function bar(baz) { // baz defined in local scope of bar     // innerfunction defined in local scope of bar    // hence has access baz , foo    function innerfunction() {        console.log(foo, baz);    } } 

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applied example:

in scenario 1, callbackfn defined in scope (global) a, b , classobj (and callbackfn itself) defined. hence inside callbackfn, a, b, classobj, param3 , param4 accessible.

in scenario 2, callback function defined in scope (somememberfunction) a, b, classobj visible , param1 , param2 defined. hence inside callback, a, b, classobj, param1, param2, param3 , param4 accessible.