Recursive call in Asynchronous function in node.js

Question

I have written one recursive query to update data in database level wise or hierarchical manner.

I have written this code :

let updateInvitationRewards = function(parent, level, callback) {

  Service.customer.getOneCustomer({
    _id: parent
  }, {}, {}, function(err, data) {

    console.log(data.email);
    level--;
    if (data.parent) {
      parent = data.parent.id;
    } else if (level == 0 || data.parent == null)
      callback("end");

    updateInvitationRewards(parent, level--, callback);
  });

};

updateInvitationRewards(parent, level, function(string) {
  console.log(string);
  cb();
});

This query is giving error like this :

Error: Callback was already called.

Please help me I am new to these type of callbacks.

Can you try inserting updateInvitationRewards(parent, level--, callback); inside the if(data.parent) condition? — Shriram Manoharan
– Shriram Manoharan, Commented Jun 29, 2016 at 13:33
I have not removed insertion code from this portion of code for data clarity...but this much of code is giving error... Callback was already called..I am not getting it .. — Udit Kumawat
– Udit Kumawat, Commented Jun 29, 2016 at 13:36
The error comes when you call callback function more than once. In your code, when the level becomes 0 or when parent is null, the callback is called. But your code will continue to call the udpateInvitationRewards even when the else if condition is satisfied. So by the next time the parent is null, callback will be called for the second time. This could throw error. — Shriram Manoharan
– Shriram Manoharan, Commented Jun 29, 2016 at 13:46

Jérôme · Accepted Answer · 2016-06-29 13:47:50Z

2

With all brackets, we have :

if (data.parent) {
  parent = data.parent.id;
} else if (level == 0 || data.parent == null) {
  callback("end");
}

updateInvitationRewards(parent, level--, callback);

if level equals 0 or data.parent is null, the callback is trigger, then updateInvitationRewards with the same callback.

To fix it :

if (data.parent) {
  parent = data.parent.id;
} else if (level == 0 || data.parent == null) {
  return callback("end"); // the execution stops here
}

updateInvitationRewards(parent, level--, callback);

answered Jun 29, 2016 at 13:47

Jérôme

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1 Comment

Udit Kumawat Over a year ago

Thank u very much :)

Shriram Manoharan · Accepted Answer · 2016-06-29 13:54:18Z

On any function the Callback should be called only once. If the callback is called more than once the Error: Callback was already called. would come.

From your code sample, it could be seen that the function will be recursively called. So the error could appear when the level becomes 0 or when the parent is null.

Consider the following scenario, when the level is null or the parent is null for the first time the callback will be called. But the recursive call will continue to happen. Suppose in the next execution, if the parent is null the callback will be called for the second time and this throws the error.

From the code, I couldn't see what is the use of level, but I would suggest the following code to be implemented, which makes sure that the callback isn't called for the next time.

let updateInvitationRewards = function(parent, level, callback) {

    Service.customer.getOneCustomer({
        _id: parent
    }, {}, {}, function(err, data) {
        console.log(data.email);
        level--;
        if (data.parent) {
            parent = data.parent.id;
            updateInvitationRewards(parent, level--, callback);
        } else if (level == 0 || data.parent == null)
            callback("end");
    });
};

updateInvitationRewards(parent, level, function(string) {
    console.log(string);
    cb();
});

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