How to make promise from web-socket API? #21

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1. shama commented on Sep 26, 2018

   

   shama

   on Sep 26, 2018

   Last edited by shama

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   There is a non-standard API Promise.denodeify to help convert Node.js compatible callbacks to promises but that API signature in your example doesn't match the Node.js standard of error first in callbacks.

   But you can just wrap it in a promise to turn it into a promise:

   const Market = require('some-api');
   const promise = new Promise((resolve, reject) => {
     const myMarket = new Market();
     myMarket.websockets.trades(request[1], (trades, error) => {
       if (error) {
         reject(error);
         return;
       }
       if (this.streamStatus === 1) {
         resolve(JSON.stringify(['TR', this.getTimeString(), trades]));
       } else {
         reject(new Error('This promise wasnt handled'));
       }
     });
   });
   
   promise
     .then((json) => { /* do stuff with json */ })
     .catch((err) => { /* do stuff with err */ })
2. allawitte commented on Sep 26, 2018

   

   allawitte

   on Sep 26, 2018 via email

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   Thank you for your answer! Yes, I also tried to write exactly this type of code in first place. And I check it now.  It works very interesting - I get an expected string in the promise.then and nothing else, when  this line: myMarket.websockets.trades(request[1], (trades, error) => {  ... } - it is websocket, I should get such lines every few seconds or milliseconds. Do you have an idea how handle it? Best, Alla Web-site: http//allawitte.nl  

   …

   ----- Reply to message ----- Subject: Re: [shama/letswritecode] How to make promise from web-socket API? (#21) Date: 26 сентября 2018 г., 19:09:47 From: Kyle Robinson Young <notifications@github.com> To: shama/letswritecode <letswritecode@noreply.github.com> There is a non-standard API Promise.denodeify to help convert Node.js compatible callbacks to promises but that API signature in your example doesn't match the Node.js standard of error first in callbacks. But you can just wrap it in a promise to turn it into a promise: const Market = require('some-api'); const promise = new Promise((resolve, reject) => { const myMarket = new Market(); myMarket.websockets.trades(request[1], (trades, error) => { if (error) { reject(error); return; } if (this.streamStatus === 1) { resolve(JSON.stringify(['TR', this.getTimeString(), trades])); } else { reject(new Error('This promise wasnt handled')); } }); }); promise .then((json) => { /* do stuff with json */ }) .catch((err) => { /* do stuff with err */ }) — You are receiving this because you authored the thread. Reply to this email directly, view it on GitHub, or mute the thread.  
3. shama commented on Sep 26, 2018

   

   shama

   on Sep 26, 2018

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   Ah ha, I see. If (trades, error) => {} gets called multiple times then it's effectively an Event Emitter. In which case you shouldn't use a promise here. Promises are intended to be resolved a single time so they're not a very good API for events.