Node.JS: How to pass variables to asynchronous callbacks? [duplicate]

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:

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'.

JohnnyHK · Accepted Answer · 2012-11-04 19:43:31Z

50

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]);
}

edited Nov 4, 2012 at 19:43

answered Nov 4, 2012 at 19:11

JohnnyHK

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4 Comments

slebetman Over a year ago

It's not about scope. It's about closures. Lots of other languages don't have block scope yet don't face this issue due to lack of closures.

JohnnyHK Over a year ago

The point is that if JavaScript did support block scope, then the closure access to url in the callback function of the OP's code would work because each iteration of the loop would get its own url variable (like in C#).

Marc Over a year ago

The require is just in there for brevity, in my code all requires are at the beginning.

Aero Wang Over a year ago

If I don't use forloop within the callback it will return url undefined.

Tjs · Accepted Answer · 2016-05-01 20:50:19Z

13

Check this blog out. A variable can be passed by using .bind() method. In your case it would be like this:

var links = ['http://google.com', 'http://yahoo.com'];
for (link in links) {
var url = links[link];

require('request')(url, function() {

    console.log(this.urlAsy);

}.bind({urlAsy:url}));
}

answered May 1, 2016 at 20:50

Tjs

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2 Comments

Levi Roberts Over a year ago

I like this method more as it's much less verbose than wrapping something in a function.

Aero Wang Over a year ago

This one works without forloop!

Community · Accepted Answer · 2017-05-23 11:46:38Z

7

See https://stackoverflow.com/a/11747331/243639 for a general discussion of this issue.

I'd suggest something like

var links = ['http://google.com', 'http://yahoo.com'];

function createCallback(_url) {
    return function() {
        console.log(_url);
    }
};

for (link in links) {
    var url = links[link];
    require('request')(url, createCallback(url));
}

edited May 23, 2017 at 11:46

CommunityBot

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answered Nov 4, 2012 at 19:15

BobS

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1 Comment

slebetman Over a year ago

There are lots of better links to give on StackOverflow to explain the issue. This one for example: stackoverflow.com/questions/3572480/…

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