Asynchronous Javascript Variable Overwrite

The code has an issue that the variable gets over written when the asynchronous function is called. How can it be fixed?

Code:

for (x in files) {

 asynchronousFunction(var1, var2, function(){
      console.log(x.someVaraible);
  }); 

}

Now the issue is that when the callback function in the asynchronousFunction is called the x.files variable has been updated to the next varaible in the json array files. I want that the variable should contain the previous value.

The number of variables in the callback function can not be changes, so variable name can not be sent in the call back function.