How to update the value of a variable within a callback?

Question

I am creating a module with Nodejs and in this module I created a function that returns a value, being this value updated within a function of type callback. The problem is that this variable/value is not being updated. I would like to know how to solve the problem. I’m a beginner in Nodejs and I researched it, but I couldn’t find the solution to my problem. Below the code, the variable I refer to is "output", declared at the beginning of the function:

exports.sendMessage = function (chat, message, workspaceId){

    let output = "";

    chat.message(
        {
            input: { text: message },
            workspace_id: workspaceId
        },
        function (error, response){
            if(error){
                console.error(error);
            } else {
                output = response;
                console.log(JSON.stringify(response, null, 2));
            }
        }
    );

    return output;
}

Answer 1

You can’t use returns for this. When the function sendMessage returns, the function you associated with chat.message may not even have run yet, because all this is asynchronous.

There where you did output = response, you should remove this and make a function call that will continue the work for you, because only there is that you are sure that has the value of response correct.

For example:

processSendMessageResponse = function(response) {
    // fazer alguma coisa com a resposta.
}

exports.sendMessage = function (chat, message, workspaceId){

    let output = "";

    chat.message(
        {
            input: { text: message },
            workspace_id: workspaceId
        },
        function (error, response, output){
            if(error){
                console.error(error);
            } else {
                processSendMessageResponse(response)
                console.log(JSON.stringify(response, null, 2));
            }
        }
    );

}

Another option is the function sendMessage receive by parameter, along with the three it already receives, the function itself that will be called when it ends. For example:

exports.sendMessage = function (chat, message, workspaceId, messageSentCallback){

    let output = "";

    chat.message(
        {
            input: { text: message },
            workspace_id: workspaceId
        },
        function (error, response, output){
            if(error){
                console.error(error);
            } else {
                messageSentCallback(response);
                console.log(JSON.stringify(response, null, 2));
            }
        }
    );

}

and the use of it could be so:

processSendMessageResponse = function(response) {
    // fazer alguma coisa com a resposta.
}

sendMessage(chat, message, workspaceId, processSendMessageResponse);

or, even simpler:

sendMessage(chat, message, workspaceId, function(response) {
    // fazer alguma coisa com a resposta.
});

I recommend you read more about asynchronous functions to get the paradigm right, as it is essential to work well with Nodejs and Javascript in general.