JavaScript function within a function - returning an object

Question

I would like to call a function within a function and have this inner function return an object. I am using a JSONRequest function that I created myself so just assume that the request fetches an array of roles. Here is the code:

(users = function(){

    this.getUserRoles = function(){

        var params = {};

        var json = new JSONRequest(function(data){

            rolesObj = data['roles'];

            return rolesObj;

        }, 'get_roles', params);

    }

});

Then I call the following but it returns undefined:

var cls = new users();

alert(cls.getUserRoles().length);

Matt Wonlaw · Accepted Answer · 2012-07-18 03:43:49Z

ajax requests are asynchronous so you're not going to get a return value from them. You do, however, receive a callback when they complete so you can pass your "return" value into a callback.

Here is an example of your code re-written to use a callback:

this.getUserRoles = function(completionCallback){

    var params = {};

    var json = new JSONRequest(function(data){

        rolesObj = data['roles'];

        completionCallback(rolesObj);
        return rolesObj;

    }, 'get_roles', params);

}

and then using it:

var cls = new users();
cls.getUserRoles(function(roles) {
    alert(roles.length);
});

The rolesObj is passed into the callback function once the JSON request completes.

thank you! worked like a charm and now I can customize what to do with the data by having unique callback functions. awesome.

Ned Batchelder · Accepted Answer · 2012-07-18 03:32:00Z

1

Your getUserRoles function doesn't return anything. It invokes an asynchronous JSON request, and the success callback for that request returns something.

There is no way to make an asynchronous request in a function and have the function synchronously return a value from the request. The request will take an unknown amount of time, and the requesting function will return as soon as the request has been sent. The value won't have arrived back from the server yet.

answered Jul 18, 2012 at 3:32

Ned Batchelder

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1 Comment

Freddie Over a year ago

hmm... then what is the best way to make certain repeatable functions modular? For example, I need to get a list of roles across different aspects of my web application and use it in different ways. I might want to get a list of roles and put it in a select box, or get that list and put it in a div...

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