如何将两个呼叫的响应合并为一个响应？

Question

I'm getting an empty response ( {} ), whereas my expected response is of format:

{
    locationResponse: "location foo",
    forecastResponse: "forecast bar"
}

In my index file I have:

const {getCity} = require('./routes/city');
const {getForecasts} = require('./routes/forecast');

app.get('/forecasts', function (req, res) {
    var location = getCity(req, res);
    var forecast = getForecasts(req, res);

    //these are logged as undefined
    console.log("Inside index.js");
    console.log(location);
    console.log(forecast);
    res.send({locationResponse: location, forecastResponse: forecast});
});

Inside forecast file I have the following, and a similar one is in city file:

module.exports = {
    getForecasts: (req, res) => {
       var result = //mySQL DB calls and processing
       console.log("Inside getForecasts");
       console.log(result); //actual result printed
       return "Forecast";
    }

UPDATE : So I added some logs right before each call's return statements and figured out that the logs are printed in the following order, which means, it is not working as expected because I have not considered the fact that they are asynchronous calls.

Inside index.js
undefined
undefined
Inside getForecasts
{result}

Answer 1

The problem here is that in your ./routes/forecast/ getForecasts method, you're telling the response to send, with the data "Forecast". You should only ever use res.send once per request, as this will resolve the response and return to the client.

Instead, your getForecasts method should just return whatever data you need, and your index file should handle the response. If you need getForecasts to handle a response too, perhaps because you're sending requests directly to a forecasts endpoint that doesn't require location data, then you can refactor your code so that both index and forecasts make a call to get the data you need. For example:

/* index.js */
const {getCity} = require('./data/city');
const {getForecasts} = require('./data/forecast');

app.get('/forecasts', function (req, res) {
    var location = getCity();
    var forecast = getForecasts();
    res.send({locationResponse: location, forecastResponse: forecast});
});

/* data/forecast.js */
module.exports = {
    getForecasts: () => {
        return "Forecast";
    }
};

/* data/city.js */
module.exports = {
    getCity: () => {
        return "City";
    }
};

Then you can also have:

/* routes/forecast.js */
const {getForecasts} = require('../data/forecast');
module.exports = {
    getForecasts: (req, res) => {
        res.send(getForecasts());
    }
};

The above may be overcomplicating things, but I made the assumption that if you're using a routes directory, you probably want route handlers to be stored there. Hope this helps.

Answer 2

Seems both of your getCity() and getForecasts() functions are async. These asynchronous functions return a promise rather actual response.

So you can use simple asysn/await or Promise.all in JS to solve the issue.

Option 1: Use await for the promise to resolve before logging the message to the console:

app.get('/forecasts', async function (req, res) {
    var location = await getCity(req, res);
    var forecast = await getForecasts(req, res);

    //these are logged as undefined
    console.log("Inside index.js");
    console.log(location);
    console.log(forecast);
    res.send({locationResponse: location, forecastResponse: forecast});
});

Option 2: Use Promise.all() to wait for all the promises to have fulfilled.

app.get('/forecasts', function (req, res) {
    var list = await Promise.all([getCity(req, res), getForecasts(req, res)]);

    //these are logged as undefined
    console.log("Inside index.js");
    console.log(list[0]);
    console.log(list[1]);
    res.send({locationResponse: list[0], forecastResponse: list[1]});
});

Answer 3

You can make use of async/await syntax.

app.get('/forecasts', async function (req, res) {
    var location = await getCity(req, res);
    var forecast = await getForecasts(req, res);

    //these are logged as undefined
    console.log("Inside index.js");
    console.log(location);
    console.log(forecast);
    res.send({locationResponse: location, forecastResponse: forecast});
});