wait for async to complete before return

Question

wait for async to complete before return

mongoosejs async code .

    userSchema.static('alreadyExists',function(name){
    var isPresent;
        this.count({alias : name },function(err,count){
        isPresent = !!count
        });
    console.log('Value of flag '+isPresent);
    return isPresent;
});

I know isPresent is returned before the this.count async function calls the callback , so its value is undefined . But how do i wait for callback to change value of isPresent and then safely return ?

what effect does (function(){ asynccalls() asynccall() })(); has in the async flow . What happens if var foo = asynccall() or (function(){})() Will the above two make return wait ?

can process.nextTick() help?

I know there are lot of questions like these , but nothing explained about problem of returning before async completion

node.js
asynchronous
mongoose

Answer 1

There is no way to do that. You need to change the signature of your function to take a callback rather than returning a value.

Making IO async is one of the main motivation of Node.js, and waiting for an async call to be completed defeats the purpose.

If you give me more context on what you are trying to achieve, I can give you pointers on how to implement it with callbacks.

Edit: You need something like the following:

userSchema.static('alreadyExists',function (name, callback) {
    this.count({alias : name}, function (err, count) {
        callback(err, err ? null : !!count);
        console.log('Value of flag ' + !!count);     
    });
});

Then, you can use it like:

User.alreadyExists('username', function (err, exists) {
    if (err) {
        // Handle error
        return;
    }

    if (exists) {
        // Pick another username.
    } else {
        // Continue with this username.
    }
}