Javascript (NodeJS) Callback Scope

Question

How do you access parent scope from a callback. The callback is function (err, obj). The var to_user_id is the same in all iterations. It looks like the callbacks are processed after all the iterations are done so the var to_user_id is only one value for all callbacks.

for(var i = 0, len = keys.length; i < len; i++) {

  to_user_id = keys[i].replace('m', '')

  client.get(keys[i], function (err, obj) {
    //var not updating, why is both to_user_id=77
    console.log("match: to_user_id=" + to_user_id + " from_user_id=" + obj)
    var match = "match: to_user_id=" + to_user_id + " from_user_id=" + obj
    io.emit(1, match);
  });

}

Output

See how to_user_id is 77 for both iterations. One should be 6 and the last should be 77.

match: to_user_id=77 from_user_id=77

match: to_user_id=77 from_user_id=6

client.get is a redis function just in case you're wondering.

mscdex · Accepted Answer · 2014-11-08 19:47:50Z

1

Your client.get() calls are asynchronous. As such, the for-loop finishes before any of the client.get() callbacks are executed. That means that to_user_id will be set to keys[keys.length - 1].replace('m', '') by the time the first client.get() callback is executed. So that is the reason why you are seeing the same to_user_id in your output.

The fix here is to use a closure to capture the current value of to_user_id. The simplest way of doing this is to use keys.forEach():

keys.forEach(function(key) {
  to_user_id = key.replace('m', '')

  client.get(key, function (err, obj) {
    console.log("match: to_user_id=" + to_user_id + " from_user_id=" + obj)
    var match = "match: to_user_id=" + to_user_id + " from_user_id=" + obj
    io.emit(1, match);
  });
});

answered Nov 8, 2014 at 19:47

mscdex

107k15 gold badges201 silver badges159 bronze badges

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

2 Comments

Natus Drew Over a year ago

Ahh. The closure is created inside the forEach.

Natus Drew Over a year ago

I ended up using your code because I couldn't resist how succinct it was. Javascript is almost as beautiful as Ruby.

Krzysztof Safjanowski · Accepted Answer · 2014-11-08 21:56:54Z

1

You must to create closure to save value of i variable.

for (var i = 0, len = 5; i < len; i++) {
  (function(i, to_user_id) {
    client.get(i, function(err, obj) {
      io.emit("match: to_user_id=" + to_user_id + " from_user_id=" + obj)
    });
  }(i, keys[i].replace('m', '')));
}

edited Nov 8, 2014 at 21:56

answered Nov 8, 2014 at 19:58

Krzysztof Safjanowski

7,4583 gold badges38 silver badges50 bronze badges

1 Comment

Natus Drew Over a year ago

Awesome. You're a JS genius.

Collectives™ on Stack Overflow

Javascript (NodeJS) Callback Scope

2 Answers 2

2 Comments

1 Comment

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

2 Answers 2

2 Comments

1 Comment

Your Answer

Sign up or log in

Post as a guest

Related