CODEWITHSUNDEEP
javascriptnode.jserror-handlingpromisecallback
Nat
Nat

Reputation: 1762

How to handle errors from within a callback in Nodejs

I have this code that invokes a function and has a callback with error and data parameters:

app.get('/lights', (req,res) => {
    hue.getLights(function(err, data){
        if(err) res.status(401).send("An error occured: ", err.message);
        res.send(data);
    });
})

The function that it invokes is:

let getLights = function(callback){
    fetch(`http://${gateway}/api/${username}/lights`, {
        method: 'GET'
    }).then((res) => {
        if(res.ok){
            return res.json();
        }else{
            throw new Error(res.message);
        }
    }).then((json) => {
        lightsArray = []
        for (var i in json){
            lightsArray.push(`ID: ${i} Name: ${json[i]['name']}`);
        }
        return callback(lightsArray);
    });
}

When I make an error occur, the error isn't caught, nor is any error displayed, the app crashes with the message: UnhandledPromiseRejectionWarning: Unhandled promise rejection. This error originated either by throwing inside of an async function without a catch block, or by rejecting a promise which was not handled with .catch().

Now I know I'm missing a lot, this is my first time using callbacks, let alone handling errors.

Could someone help me with making the error callback work, also show me some flaws in what I'm doing, as I know this won't catch every error that may happen, only errors caused by using the fetch function.

Thanks!

This is my other function (similar but uses a catch aswell, which I think I have done incorrectly too):

let getLightDetails = function (ID, callback) {
    fetch(`http://${gateway}/api/${username}/lights/${ID}`, {
        method: 'GET'
    }).then((res) => {
        if(res.ok){
            return res.json();
        }else{
            throw new Error(res.message);
        }
    }).then((json) => {
        return callback(json);
    })
    .catch((err) => {
        console.log(err);
        return callback(err.message);
    });
}

Upvotes: 0

Views: 846

Answers (1)

blex
blex

Reputation: 25634

Mixing callbacks and promises can make your code a bit messy. I would stick to promises:

app.get('/lights', (req, res) => {
  return hue.getLights()
    .then(data => {
      res.send(data);
    })
    .catch(err => {
      res.status(401).send("An error occured: ", err.message);
    });
})

and hue.js

const fetch = require('node-fetch');
const gateway = "192.168.0.12";
const username = "username-A";

function fetchAPI(url, ...rest) {
    return fetch(`http://${gateway}/api/${username}${url}`, ...rest);
}

function getLights() {
    return fetchAPI(`/lights`)
    .then(res => res.json())
    .then(json => json.map((light, i) => `ID: ${i} Name: ${light.name}`));
}

function getLightDetails(id) {
    return fetchAPI(`/lights/${id}`)
    .then(res => res.json());
}

function getLightState(id) {
    return fetchAPI(`/lights/${id}`)
    .then(res => res.json())
    .then(light => `Name: ${light.name} On: ${light.state.on}`);
}

function setLightState(id, state) {
    return fetchAPI(`/lights/${id}/state`, {
        method: 'PUT',
        body: JSON.stringify({"on": state })
    }).then(res => res.json());
}

module.exports = { getLights, getLightDetails, getLightState, setLightState };

Upvotes: 1

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