While loop promise, chain promise

Question

I have a script that should try to connect 3 times to server if this fails navigate the user to the No Connection page.

I don't know how to attempt a connection 3 times and then navigate user to the No Connection page if this fails.

How can I use my code to achieve this problem.

Here is the code I have so far:

const fetchModule = require('fetch');
const frameModule = require('ui/frame');

let timeTrig;
const waitPeriod = 7000;

function checkServerStatus() {
    if (timeTrig) {
        clearTimeout(timeTrig);
        timeTrig = null;
    }
    timeTrig = setTimeout(() => {
    frameModule.topmost().navigate('./views/no-connection');
    }, waitPeriod);
    return fetchModule.fetch(`${randomUrl}auth/ping`)
        .then((result) => {
            clearTimeout(timeTrig);
            if (result && result.ok) {
                return true;
            }
            frameModule.topmost().navigate({
                moduleName: views/no-connection,
                transition: {
                    name: 'slide',
                },
            });
        })
        .catch(() => {
            if (timeTrig) {
                clearTimeout(timeTrig);
                timeTrig = null;
            }
            frameModule.topmost().navigate({
                moduleName: 'views/no-connection',
                transition: {
                    name: 'slide',
                },
            });
        });
}

Any help is appreciated.

so, you have no while loop in the code you posted - you want to return the `fetchModule.fetch` *n* times then give up? — Jaromanda X, Apr 18 '18 at 00:01
There was a while loop but I deleted it since it run only once, I would like to loop 3 times if there is no connection, and if there is a response from server to return true :).. also Im using Nativescript as you can see — Loki, Apr 18 '18 at 00:07
not sure which part of your code you should repeat though - there's a few possibilities — Jaromanda X, Apr 18 '18 at 00:10
Im assuming the part when it returns fetchModule.fetch, or right from the beggining of the function, because Im using setTimeout if there is no response, for some period of time to navigate ot no-connection... Im not sue myself, but sure you can give it a shot :) — Loki, Apr 18 '18 at 00:18

Jaromanda X · Accepted Answer · 2018-04-18T00:34:23.283

Perhaps something like this is what you want - not exactly sure which part of your code should be retried, however this may give you some ideas

const fetchModule = require('fetch');
const frameModule = require('ui/frame');

const waitPeriod = 7000;

// retry "cont" times or until the promise resolves
const retry = (cont, fn) => fn().catch(err => cont > 0 ? retry(cont - 1, fn) : Promise.reject(err));

// failure code - reason will be the last failure reason ('not ok' or 'timeout') - not that it is used in your code
const noConnection = reason => frameModule.topmost().navigate({
    moduleName: 'views/no-connection',
    transition: {
        name: 'slide',
    },
});

// lets race between (resolved or rejected) fetch and a rejected timeout
const doFetch = () => Promise.race([
    fetchModule.fetch(`${randomUrl}auth/ping`).then(result => {
        if (result && result.ok) {
            return true;
        }
        return Promise.reject('not ok');
    }),
    // above can be written as
    // fetchModule.fetch(`${randomUrl}auth/ping`).then(result => (result && result.ok) || Promise.reject('not ok')),
    new Promise((resolve, reject) => setTimeout(reject, waitPeriod, 'timeout'))
]);

const checkServerStatus = () => {
    retry(3, doFetch).catch(noConnection)
};

Alternatively, this is the same code, just organised differently. I think better :p

const fetchModule = require('fetch');
const frameModule = require('ui/frame');

const waitPeriod = 7000;

// retry "cont" times or until the promise resolves
const retry = (cont, fn) => fn().catch(err => cont > 0 ? retry(cont - 1, fn) : Promise.reject(err));
// reject after a given timeout
const delayedReject = (delay, reason) => new Promise((_, reject) => setTimeout(reject, delay, reason));
//
const doFetch = () => fetchModule.fetch(`${randomUrl}auth/ping`)
.then(result => (result && result.ok) || Promise.reject('not ok'));

const checkServerStatus = () => retry(3, () => Promise.race([doFetch(), delayedReject(waitPeriod, 'timeout')]))
.catch(reason => frameModule.topmost().navigate({
    moduleName: 'views/no-connection',
    transition: {
        name: 'slide',
    },
}));

yeah I see, while loop was my first assumption, how I should do it :D, thanx again for fast response — Loki, Apr 18 '18 at 00:28

score 1 · Answer 2 · answered Apr 18 '18 at 09:02

This "try n times and then fail" type of questions are frequently appearing, let in the form of http fetches or web sockets and what not. I had actually implemented a promise TryNTimes functionality before.

Here is a more interesting version which i think is very elegant and reusable.

We have several utility functions just to mimic the fetch requests and promise sequencing.

The following bring function is like fetch with 50% probability of resolving.

bring    = (url,i) => new Promise((v,x) => Math.random() > 0.50 ? v(`Data from ${url}`) : x(`Error @ promise # ${i}`))

Next we set up an size n array of such bring functions.

promises = (f,n) => Array(n).fill(f),

Next we sequence the promise returning bring functions recursively. If the promise returned by the invoked bring function resolves we return that resolution but if rejects then we carry on with the next one in the array up until there are no more left. Obviously the size of the array is the try count.

var bring    = (url,i) => new Promise((v,x) => Math.random() > 0.50 ? v(`Data from ${url}`) : x(`Error @ promise # ${i}`)),
    promises = (f,n) => Array(n).fill(f),
    tryProms = ([f,...fs], url, i = 1) => f(url,i).then(v => `${v} @ try # ${i}`,
                                                        e => (console.log(`Failed @ try #${i}`),
                                                              fs.length ? tryProms(fs,url,++i)
                                                                        : `All ${i} tries are failed`));
tryProms(promises(bring,3),"http://www.someAPI/data").then(console.log, console.log);

While loop promise, chain promise

2 Answers2

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