When using callbacks inside a loop in javascript, is there any way to save a variable that's updated in the loop for use in the callback?

Question

Let's say I have something as follows:

for(var i = 0; i < length; i++){
  var variable = variables[i];
  otherVariable.doSomething(variable, function(err){ //callback for when doSomething ends
    do something else with variable;
  }

By the time the callbacks are called, variable will inevitably be the last variable for all the callbacks, instead of being a different one for each callback, as I would like. I realize that I could pass variable to doSomething() and then get that passed back as part of the callback, but doSomething() is part of an external library, and I'd rather not mess around with the source code for that.

Do those of you that know JavaScript better than I do know if there are any alternative ways to do what I'd like to do?

Best, and thanks,
Sami

Answer 1

A common, if ugly, way of dealing with this situation is to use another function that is immediately invoked to create a scope to hold the variable.

for(var i = 0; i < length; i++) {
  var variable = variables[i];
  otherVariable.doSomething(function(v) { return function(err) { /* something with v */ }; }(variable));
}

Notice that inside the immediately invoked function the callback that is being created, and returned, references the parameter to the function v and not the outside variable. To make this read much better I would suggest extracting the constructor of the callback as a named function.

function callbackFor(v) {
  return function(err) { /* something with v */ };
}
for(var i = 0; i < length; i++) {
  var variable = variables[i];
  otherVariable.doSomething(callbackFor(variable));
}