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Tapas Adhikary

Educator @tapaScript | Founder CreoWis & ReactPlay - Writer - YouTuber - Open Source

Sep 20, 2021

6 Common mistakes in using JavaScript Promises

Hello friends 👋, we are reaching the end of the series, Demystifying JavaScript Promises - A New Way to Learn. So far, we have learned a lot about JavaScript asynchronous programming and promises. If you are new to the series, please check out the p...

blog.greenroots.info8 min read

#javascript #codenewbies #nodejs #beginners

Responses8

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For such things like loops, filters, groups I use alot library - it designed for lazy iterations over arrays, but also supports Promise handlers. The first example would look like this:

const names = ['saviomartin', 'victoria-lo'];
const users = await alot(names)
    .mapAsync(async name => {
        let { data: user } = await axios.get(`api.github.com/users${name}`);
        return user;
    })
    .toArrayAsync({ threads: 4 })

M. Suman Patra

Developer

Sep 21, 2021

Really Nice article !!

I will take this opportunity to clear my doubts. I code as per below.

const request = require('request');
const test = userData => {
  console.log('test::parameters', userData);
  if (userData && userData.request === true) {
    const option = {
      method: 'POST',
      url: 'jsonplaceholder.typicode.com/posts&,
      headers: {
        'Content-type': 'application/json; charset=UTF-8'
      },
      json: true,
      proxy: null,
      body: {
        title: 'foo',
        body: 'bar',
        userId: 1
      }
    };
    console.log('test::outgoingBody::', option);
    return new Promise((resolve, reject) => {
      request(option, (error, response) => {
        if (error) {
          return reject(error);
        } else {
          if (response && response.body ) {
            return resolve();
          } else {
            return reject(new Error('Error Response'));
          }
        }
      });
    });
  } else {
    return Promise.reject(new Error('Invalid Userdata'));
  }
};
//some business logics
test().catch(err=>console.error(err))
//some business logics
//--------------------------
//some business logics
test({request:true}).catch(err=>console.error(err))
//some business logics

As I am not returning anything from test function, I don't use then callback.
I use Promise.reject to throw any synchronous error.

Am I doing correctly?

Tapas Adhikary, To catch test function errors gracefully, I am using catch callback but I thought then callback is not needed because I am not waiting for test function execution and also not returning anything from API call/from the test function.

Note: As the request dependency does not support async/await, I had to use promise constructor.

Tapas Adhikary

Educator @tapaScript | Founder CreoWis & ReactPlay - Writer - YouTuber - Open Source

Sep 21, 2021

M. Suman Patra, I get that. In your typical case, where you do not return anything by resolving the promise, you are not accountable for handling it.

if (response && response.body ) {
     return resolve();
}

So, the current handling works to take care of the error test cases alone. If that's what is the intention, it looks good to me.

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6 Common mistakes in using JavaScript Promises

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