— Jul 30, 2019· 3 mins to read

Promises inside a loop - Javascript ES6

In this article, we will see how to write a promise function inside a loop. Promises inside a loop - Javascript ES6

If you want to learn more about Javascript. check out this articles

Prototypal Inheritance - Javascript Weekly

Understanding Closures in Javascript - Javascript Weekly

Before jumping into the code, we will see why we need to write a promise function inside a loop.

Scenario

Consider a situation where you need to send mail to list of emails. you will write a function which can send a single email at once.

Let’s say that you have a list of emails like this

const emailList = ["ganesh@gmail.com", "john@gmail.com", "tarry@gmail.com"];

Firstly, you write a single function which takes an emailId and send a mail to the corresponding mail.

function sendMail() {
  //Sending mail
}

you need to call the function as per the count of emails you have in the array.

you can think of, why can’t I call the function inside the for a loop. it will be simple ..right??.

No… that’s not going to work. because sending a mail is an asynchronous function. Loop will not wait until the previous function completes

So, there might be a chance of missing a function call or sending mail. To solve this problem, we need to use Promise.

Promises inside a loop - Javascript ES6

Firstly, write the email sending logic inside a function which returns a Promise.

const sendEmail = (userEmail) => {
  return new Promise(async (resolve, reject) => {
    //this is a mock email send logic
    setTimeout(() => {
      resolve(`Email Sent to ${userEmail}`);
    }, 3000);
  });
};

call the function inside the Javascript map() function and Promise.all

const sendEmails = async () => {
  const userEmails = ["ganesh@gmail.com", "john@gmail.com", "Sam@gmail.com"];

  const status = await Promise.all(userEmails.map((email) => sendEmail(email)));

  console.log("Status =>", status);
};

This is one of the ways with which you can call promise inside loops.

To sum up, Promises are one the powerful addition in Javascript ES6.Promises inside a loop - Javascript ES6

In addition, we will see more about Core Concepts of Javascript in upcoming articles. Stay Tuned.

Until then Happy Coding :-)