Callback for Async.Map Not Working - node.js

Question

I'm relatively new to callbacks and have been unsuccessful in getting the following code to work. I have used the async.map function to return the data from each web call to its respective page. However, my console.log(return) is returning [ , undefined] even though the console.log(data) prior to the callback(data) is returning the web page's html. Here's my code:
var http = require("http"),
fs = require("fs"),
page, chap, ot,
async = require("async");
ot = fs.open('ot.txt', 'w');
page = "test";
chap = 2;
function getData(url, callback) {
var data = "";
var options = {
host: "rs.au.com",
port: 80
}
options.path = url;
console.log("request sent to: http://" + options.host + options.path);
var req = http.request(options, function(res) {
console.log("Response received " + res.statusCode);
res.on('data', function(chunk) {
data += chunk;
});
res.on('end', function(e) {
console.log(data);
callback(e, data);
});
}).end();
}
function main() {
var pathArr = [];
for ( var i = 1; i <= chap; i++ ) {
pathArr[i] = "/".concat(page, "/", i, ".html");
}
async.map(pathArr, getData, function(err, result) {
console.log("The result is :" + result);
});
}
main();
Could anyone please point out why my code is not working and how I can correct it?
Much appreciated!
EDIT: After Brandon Tilley's response I amended the callback function from callback(data) to callback(e, data), however I'm getting no response now from the last console.log output.

Answer 1

The Async library assumes your callbacks adhere to the standard Node.js callback signature, which is callback(err, others...). Since you are passing data as the first argument, Async assumes it's an error. You should use callback(e, data) instead (since e will be null in the case of no errors).
[Update]
The other problem is that your array is not correct. Since i starts at 1 and goes up to chap, pathArr[0] is undefined. Change:
pathArr[i] = "/".concat(page, "/", i, ".html");
to
pathArr[i-1] = "/".concat(page, "/", i, ".html");