Async - passing variables and preserving context

Question

Async - passing variables and preserving context

If you have the following code :

var asyncConfig = {};
var a, b;
for(var i = 0; i < someValue; i++) {
    // do something with a
    // do something with b
    asyncConfig[i] = function(callback) {
        func(a, b, callback); // func is async
    }
}
// Include some more parallel or series functions to asyncConfig
async.auto(asyncConfig);

How can you pass the values of the variables a and b to func so that when async.auto(asyncConfig) is executed after the for loop, the context of a and b is preserved ?

(Different context of a and b for every execution of func.)

Thank you in advance !

javascript
node.js
asynchronous

Answer 1

var asyncConfig = {};
var a, b;
for(var i = 0; i < someValue; i++) {
    // do something with a
    // do something with b
    (function(a,b){
      asyncConfig[i] = function(callback) {
        func(a, b, callback); // func is async
      }
    })(a,b);
}
// Include some more parallel or series functions to asyncConfig
async.auto(asyncConfig);

Answer 2

possible alternative using bind :

var asyncConfig = {};
var a, b;
for(var i = 0; i < someValue; i++) {
    // do something with a
    // do something with b
    asyncConfig[i] = func.bind(asyncConfig, a, b);
}
// Include some more parallel or series functions to asyncConfig
async.auto(asyncConfig);

Make sure to check if the environments where you use this support bind. Also, I am binding the "this" value to asyncConfig, this may not be appropriate for you.

edit : Reading over the question again, are a and b primitives or objects/arrays? If they aren't primitives, then you'll want to clone them.