How do I do async work in a function and then return a value from that function when finished?

Question

I am using a library that requires me to pass in a "validate" function. In this function I need to validate some input and then return true if it passes or false if it fails.

The problem is that I need to check if a user exists in the database for this validation function and Mongoose invokes a callback with the result. If I return within the callback it's just returning from the callback function not my validate function.

...
validate: function (username) {
    User.findOne({ username: username }, function (err, user) {
        if (user)
            // User exists so we should return false from the validate function
        else
            // User does not exist so we should return true from validate.
    });
}

I'm trying to use the async library to solve this, but async also invokes a callback when its done!

round peg in a square hole, won't work. you can fake async, but you can't fake sync. — dandavis
– dandavis, Commented Sep 26, 2013 at 20:28

Brad Christie · Accepted Answer · 2013-09-26 20:28:28Z

2

Short answer: You can't.

For all intents and purposes JavaScript is single-threaded. Yes, you have callbacks and presence of multi-threading (setTimeout, setInterval, event callbacks) but you can't jump out of a callback and come back in the same function.

My advice is to try to pre-process this validation before validate is called so you have a "yay or ney" result to work within within validate.

answered Sep 26, 2013 at 20:28

Brad Christie

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2 Comments

ChevCast Over a year ago

Dang. I don't have the value to search for in the database until this function is called. In what way should the library be modified so that this work can be done? I will modify it and submit a pull request if I can but I'm not sure how I would change this behavior. Should one of the arguments passed to the validate function be a done callback or something?

Brad Christie Over a year ago

I don't know the work-flow, but you either perform the lookup while (or just after) the value is being changed (so at time of validation you have a checksum value), or introduce a pre-step that the user needs to execute before validate is available. (I know you're in node, but a for instance would be the submit button isn't enabled until the user presses "check" next to the username field)

Sriharsha · Accepted Answer · 2013-09-26 21:04:30Z

0

You can use promises to achieve this.

var q = require('q');

validate: function (username) {
    var deferred = q.defer();
    User.findOne({ username: username }, function (err, user) {
      if (err){return deferred.reject(err)}
      if (user)
        // User exists so we should return false from the validate function
        deferred.resolve(false);
      else
        // User does not exist so we should return true from validate.
        deferred.resolve(true)
    });
    return deferred.promise;
 }

and then use the validate function as below.

validate('username').then(function(result){
  if (result)
     console.log('user doesnt exist');
  else
     console.log('user exists');

}, function(err){
   if(err)
      console.log('Error', err); 
})

answered Sep 26, 2013 at 21:04

Sriharsha

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1 Comment

ChevCast Over a year ago

I can't return a promise. The library expects true or false to be returned from the validate function.

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