Node.JS: How to pass variables to asynchronous callbacks?

Question

I'm sure my problem is based on a lack of understanding of asynch programming in node.js but here goes.

For example: I have a list of links I want to crawl. When each asynch request returns I want to know which URL it is for. But, presumably because of race conditions, each request returns with the URL set to the last value in the list.

var links = ['http://google.com', 'http://yahoo.com'];
for (link in links) {
    var url = links[link];
    require('request')(url, function() {
        console.log(url);
    });
}

Expected output:

http://google.com
http://yahoo.com

Actual output:

http://yahoo.com
http://yahoo.com

So my question is either:

1. How do I pass url (by value) to the call back function? OR
2. What is the proper way of chaining the HTTP requests so they run sequentially? OR
3. Something else I'm missing?

PS: For 1. I don't want a solution which examines the callback's parameters but a general way of a callback knowing about variables 'from above'.

Answer 1

Your url variable is not scoped to the for loop as JavaScript only supports global and function scoping. So you need to create a function scope for your request call to capture the url value in each iteration of the loop by using an immediate function:

var links = ['http://google.com', 'http://yahoo.com'];
for (link in links) {
    (function(url) {
        require('request')(url, function() {
            console.log(url);
        });
    })(links[link]);
}

BTW, embedding a require in the middle of loop isn't good practice. It should probably be re-written as:

var request = require('request');
var links = ['http://google.com', 'http://yahoo.com'];
for (link in links) {
    (function(url) {
        request(url, function() {
            console.log(url);
        });
    })(links[link]);
}