Route.get() requires callback function but got a [object Promise]

Question

I am creating a REST API with express, folowing the architecture from this article . in a nutshell, a router is calling a controller.

here is an example of a call:

router.get('/', ModelsController.getModels)

This work fine so far, and now, I'm improving error handling with Boom.

I would like to use the wrapper like in this article , but as I don't use TS and as I am unfamiliar with Promises, I'm struggling with it.

Here is the wrapper:

exports.enhanceHandler = async function (handler) {
    return async function (req, res, next) {

        try {
            const result = await handler(req, res);
            if (result instanceof Error && Boom.isBoom(result)) {
                res.status(result.output.statusCode).send(formatBoomPayload(result));
            }
        } catch (error) {
            // now log errors to your errors reporting software
            if (process.env.NODE_ENV !== "production" && (error.stack || error.message)) {
                res.status(500).send(error.stack || error.message);
            } else {
                res.status(500).send(Boom.internal().output.payload);
            }
        }

        next();
    }
}

I'm trying to call it in my router, like this:

router.get('/handler', enhanceHandler(ModelsController.getModels))

However, I've got this error:

Error: Route.get() requires a callback function but got a [object Promise]

What could I do ? Do I need to resolve the promise ? modify enhanceHandler so it return a function and not a promise ?

Answer 1

Every promise object have a .then method that you need to use to get the result out of a promise object, like this:

handler(req, res).then((res) => {
    if (res instanceof Error && Boom.isBoom(res)) {
        res.status(res.output.statusCode).send(formatBoomPayload(res));
    }
});

We can also removes async from the function if we are not using await anymore.

Answer 2

Handler needs to be sync, but the function it returns can remain async.

exports.enhanceHandler = function (handler) { // delete "async" keyword
   return async function (req, res, next) {

        try {
            const result = await handler(req, res);
            if (result instanceof Error && Boom.isBoo
    ..

Answer 3

Let's see what's going on. You've called get() and for second parameter you've used enhanceHandler() call. The call of any async function returns Promise . While the get needs a function reference as the second parameter.

So first you have to avoid async keyword on a function which provides the second parameter for get().