Question

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

rated 0 times [ 11] [ 1] / answers: 1 / hits: 49354 / 12 Years ago, sun, november 4, 2012, 12:00:00

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:

How do I pass url (by value) to the call back function? OR

What is the proper way of chaining the HTTP requests so they run sequentially? OR

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'.

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mckenna

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answered 12 Years ago isaacvalentinn · Accepted Answer

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]);

}