function done in javascript

Question

Actually I am reading some codes in Nodejs, but I cannot get what it means because some common use of function that I don't quite understand. Can someone tell me what the function(done) means? It is a callback or something? in the js file I can't see the definition of done. Thanks

Andes

var getRedisUri = exports.getRedisUri = function (done) {
if (process.env.CLUSTER_CONFIG_URI) {
    return done(null, process.env.CLUSTER_CONFIG_URI);
}
//redis uri
Fs.readFile(
    '/opt/redis_uri',
    {encoding: 'utf8'},
    function (err, redis_uri_data) {
        if (err) {return done(err);}

        var redis_uri = redis_uri_data.toString();


        if (process.env.NODE_ENV !== 'production' &&
            process.env.VM_DOMAIN &&
            (redis_uri.indexOf('localhost') !== -1 || redis_uri.indexOf('127.0.0.1') !== -1)) {
            redis_uri = redis_uri.replace('localhost', process.env.VM_DOMAIN);
            redis_uri = redis_uri.replace('127.0.0.1', process.env.VM_DOMAIN);
        }

        done(null, redis_uri);
    });
};

It's just a standard Node.js callback. Do you know how callbacks work? — Qantas 94 Heavy
– Qantas 94 Heavy, Commented Nov 6, 2014 at 3:51
Hi,turrle. I can understand the callback, but I just cannot find out where is the done function. How can I find it? In IDE I cannot find it. — wiwengweng
– wiwengweng, Commented Nov 6, 2014 at 6:20

Raphael Serota · Accepted Answer · 2014-11-06 03:52:51Z

1

That line is just the start of a function definition. function(done) just means that when this function is called, it should be called with one argument, and during the execution of the function, that argument will be referred to as done.

More specifically, I think it has to do with middleware (are you using express.js here?). If you have a bunch of middleware functions in express, the express router will call those functions for you, and pass in a function as an argument which, when called, passes the request to the next middleware function. It seems like that's what's happening here, as done gets called at the end of the function.

Node.js has stuff like this because it's all asynchronous, so you can't use return statements for much.

edited Nov 6, 2014 at 3:52

answered Nov 6, 2014 at 3:47

Raphael Serota

2,21714 silver badges17 bronze badges

Sign up to request clarification or add additional context in comments.

2 Comments

wiwengweng Over a year ago

thank you Raphael. I don't think I have express.js, because I don't have this js lib in my lib. what else it may be? I cannot jump to the done var in my IDE, it just show this done is a var. no more hints.

Raphael Serota Over a year ago

It might not be obvious just from reading the code. You should read the redis documentation.

Collectives™ on Stack Overflow

function done in javascript

1 Answer 1

2 Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

2 Comments

Your Answer

Sign up or log in

Post as a guest

Related