Iterating through items and dealing with asynchronous logic (i.e. API calls) are probably two of the most common tasks we have to perform as JavaScript devs. In this article, we will discuss the best approaches to combine async/await and iterative logic. There will be a time when you would want to run async operations inside for loops (or any type of other loops). Let’s take a look at how to deal with such situations.

Update: I uploaded a video based on this article as well.

Reading Promises in sequence

Let’s say we have a list of files and we would like to read and log the contents of each file in the sequence. How would we do this? Well, we can use a for … of the loop inside an async function. Here’s the code snippet.

async function printFiles () {
  let fileNames = ['picard', 'kirk', 'geordy', 'ryker', 'worf'];
  for (const file of fileNames) {
    const contents = await fs.readFile(file, 'utf8');
    console.log(contents);
  }
}

💡 Be advised that if you want to read files in sequence you can not use a forEach loop.

Let’s elaborate on this with a quick example.

async function someFunction(items) {
  items.forEach( async(i) => {
     const res = await someAPICall(i);
     console.log('--->', res);
  });
}
function someAPICall(param) {
    return new Promise((resolve, reject)=>{
      setTimeout(()=>{
        resolve("Resolved" + param)
      },param);
    })
}
someFunction(['3000','8000','1000','4000']);

In the code above we have a simple async function called someFunction, it takes in an array as a parameter, iterates the array and for each item makes an API request (via out fake API function). In this case, we want to resolve the API calls in sequence. We want our output to print the following

// expected
3000
8000
1000
4000

Instead of this output, we see the following result

// actual
1000
3000
4000
8000

Instead of running the API calls in sequence the forEach loop just sets the API calls one after another. It doesn’t wait for the previous call to finish. This is why we get the promises that resolve first. This is the main reason we can not use a forEach loop.

On contrary, we can use a reduce function to iterate over the array and resolve the promises in sequence. Let’s take a quick look at an example of this.

function testPromise(time) {
  return new Promise((resolve, reject) => {
    setTimeout(() => {
      console.log(`Processing ${time}`);
      resolve(time);
    }, time);
  });
}

let result = [3000,2000,1000, 4000].reduce( (accumulatorPromise, nextID) => {
  return accumulatorPromise.then(() => {
    return testPromise(nextID);
  });
}, Promise.resolve());

result.then(e => {
  console.log("All Promises Resolved !!✨")
});

Pretty neat isn’t it? Another way of resolving promises in the sequence is with an async generator.

async function* readFiles(files) {
  for(const file of files) {
    yield await readFile(file);
  }
};

Generators and support by most modern browsers and Node 10 and up. You can learn more about generators and iterators in Javascript here.

Resolving Promises in parallel

Next, let’s take a look at how we can resolve promises in parallel. Let’s go back to our first example. Instead of reading the files in the sequence we now want to read them in parallel. In this scenario, we don’t care about what order the contents get printed in our console. Therefore we can simply use a Promise.all() function with a map.

async function printFiles () {
  let fileNames = ['picard', 'kirk', 'geordy', 'ryker', 'worf'];
  await Promise.all(fileNames.map(async (file) => {
    const contents = await fs.readFile(file, 'utf8');
    console.log(contents);
  }));
}

Each of the async callback function calls do return a promise, we are stashing them and resolving them all at once in parallel with a Prmiss.all().

I hope this quick read gave you an insight into how to use asynchronous code within loops. If you enjoyed this article please follow me on twitter @HaqueShadid. That’s all for today, until next time.

References:

https://developer.mozilla.org/en-US/docs/Web/JavaScript/Guide/Iterators_and_Generators

https://stackoverflow.com/questions/37576685/using-async-await-with-a-foreach-loop

https://css-tricks.com/why-using-reduce-to-sequentially-resolve-promises-works/