JavaScript async and await in loops

Zell Liew
Zell Liew
May 21, 2019 · 8 min read

Basic async and await is simple. Things get a bit more complicated when you try to use await in loops.

In this article, I want to share some gotchas to watch out for if you intend to use await in loops.

Before you begin

I'm going to assume you know how to use async and await. If you don't, read the previous article to familiarize yourself before continuing.

Preparing an example

For this article, let's say you want to get the number of fruits from a fruit basket.

const fruitBasket = {
apple: 27,
grape: 0,
pear: 14
};

You want to get the number of each fruit from the fruitBasket. To get the number of a fruit, you can use a getNumFruit function.

const getNumFruit = fruit => {
return fruitBasket[fruit];
};
const numApples = getNumFruit(“apple”);
console.log(numApples); // 27

Now, let's say fruitBasket lives on a remote server. Accessing it takes one second. We can mock this one-second delay with a timeout. (Please refer to the previous article if you have problems understanding the timeout code).

const sleep = ms => {
return new Promise(resolve => setTimeout(resolve, ms));
};
const getNumFruit = fruit => {
return sleep(1000).then(v => fruitBasket[fruit]);
};
getNumFruit(“apple”).then(num => console.log(num)); // 27

Finally, let's say you want to use await and getNumFruit to get the number of each fruit in asynchronous function.

const control = async _ => {
console.log(“Start”);
const numApples = await getNumFruit(“apple”);
console.log(numApples);
const numGrapes = await getNumFruit(“grape”);
console.log(numGrapes);
const numPears = await getNumFruit(“pear”);
console.log(numPears);
console.log(“End”);
};

With this, we can begin looking at await in loops.

Await in a for loop

Let's say we have an array of fruits we want to get from the fruit basket.

const fruitsToGet = [“apple”, “grape”, “pear”];

We are going to loop through this array.

const forLoop = async _ => {
console.log(“Start”);
for (let index = 0; index < fruitsToGet.length; index++) {
// Get num of each fruit
}
console.log(“End”);
};

In the for-loop, we will use getNumFruit to get the number of each fruit. We'll also log the number into the console.

Since getNumFruit returns a promise, we can await the resolved value before logging it.

const forLoop = async _ => {
console.log(“Start”);
for (let index = 0; index < fruitsToGet.length; index++) {
const fruit = fruitsToGet[index];
const numFruit = await getNumFruit(fruit);
console.log(numFruit);
}
console.log(“End”);
};

When you use await, you expect JavaScript to pause execution until the awaited promise gets resolved. This means awaits in a for-loop should get executed in series.

The result is what you'd expect.

“Start”;
“Apple: 27”;
“Grape: 0”;
“Pear: 14”;
“End”;

This behavior works with most loops (like while and for-of loops)...

But it won't work with loops that require a callback. Examples of such loops that require a fallback include forEach, map, filter, and reduce. We'll look at how await affects forEach, map, and filter in the next few sections.

Await in a forEach loop

We'll do the same thing as we did in the for-loop example. First, let's loop through the array of fruits.

const forEachLoop = _ => {
console.log(“Start”);
fruitsToGet.forEach(fruit => {
// Send a promise for each fruit
});
console.log(“End”);
};

Next, we'll try to get the number of fruits with getNumFruit. (Notice the async keyword in the callback function. We need this async keyword because await is in the callback function).

const forEachLoop = _ => {
console.log(“Start”);
fruitsToGet.forEach(async fruit => {
const numFruit = await getNumFruit(fruit);
console.log(numFruit);
});
console.log(“End”);
};

You might expect the console to look like this:

“Start”;
“27”;
“0”;
“14”;
“End”;

But the actual result is different. JavaScript proceeds to call console.log('End') before the promises in the forEach loop gets resolved.

The console logs in this order:

‘Start’
‘End’
‘27’
‘0’
‘14’

JavaScript does this because forEach is not promise-aware. It cannot support async and await. You _cannot_ use await in forEach.

Await with map

If you use await in a map, map will always return an array of promise. This is because asynchronous functions always return promises.

const mapLoop = async _ => {
console.log(“Start”);
const numFruits = await fruitsToGet.map(async fruit => {
const numFruit = await getNumFruit(fruit);
return numFruit;
});
console.log(numFruits);console.log(“End”);
};
“Start”;
“[Promise, Promise, Promise]”;
“End”;

Since map always return promises (if you use await), you have to wait for the array of promises to get resolved. You can do this with await Promise.all(arrayOfPromises).

const mapLoop = async _ => {
console.log(“Start”);
const promises = fruitsToGet.map(async fruit => {
const numFruit = await getNumFruit(fruit);
return numFruit;
});
const numFruits = await Promise.all(promises);
console.log(numFruits);
console.log(“End”);
};

Here's what you get:

“Start”;
“[27, 0, 14]”;
“End”;

You can manipulate the value you return in your promises if you wish to. The resolved values will be the values you return.

const mapLoop = async _ => {
// …
const promises = fruitsToGet.map(async fruit => {
const numFruit = await getNumFruit(fruit);
// Adds onn fruits before returning
return numFruit + 100;
});
// …
};
“Start”;
“[127, 100, 114]”;
“End”;

Await with filter

When you use filter, you want to filter an array with a specific result. Let's say you want to create an array with more than 20 fruits.

If you use filter normally (without await), you'll use it like this:

// Filter if there’s no await
const filterLoop = _ => {
console.log(‘Start’)
const moreThan20 = await fruitsToGet.filter(fruit => {
const numFruit = fruitBasket[fruit]
return numFruit > 20
})
console.log(moreThan20)
console.log(‘End’)
}

You would expect moreThan20 to contain only apples because there are 27 apples, but there are 0 grapes and 14 pears.

“Start”[“apple”];
(“End”);

await in filter doesn't work the same way. In fact, it doesn't work at all. You get the unfiltered array back...

const filterLoop = _ => {
console.log(‘Start’)
const moreThan20 = await fruitsToGet.filter(async fruit => {
const numFruit = getNumFruit(fruit)
return numFruit > 20
})
console.log(moreThan20)
console.log(‘End’)
}
“Start”[(“apple”, “grape”, “pear”)];
(“End”);

Here's why it happens.

When you use await in a filter callback, the callback always a promise. Since promises are always truthy, everything item in the array passes the filter. Writing await in a filter is like writing this code:

// Everything passes the filter…
const filtered = array.filter(true);

There are three steps to use await and filter properly:

1. Use map to return an array promises

2. await the array of promises

3. filter the resolved values

const filterLoop = async _ => {
console.log(“Start”);
const promises = await fruitsToGet.map(fruit => getNumFruit(fruit));
const numFruits = await Promise.all(promises);
const moreThan20 = fruitsToGet.filter((fruit, index) => {
const numFruit = numFruits[index];
return numFruit > 20;
});
console.log(moreThan20);
console.log(“End”);
};
Start[“apple”];
End;

Await with reduce

For this case, let's say you want to find out the total number of fruits in the fruitBastet. Normally, you can use reduce to loop through an array and sum the number up.

// Reduce if there’s no await
const reduceLoop = _ => {
console.log(“Start”);
const sum = fruitsToGet.reduce((sum, fruit) => {
const numFruit = fruitBasket[fruit];
return sum + numFruit;
}, 0);
console.log(sum);
console.log(“End”);
};

You'll get a total of 41 fruits. (27 + 0 + 14 = 41).

“Start”;
“41”;
“End”;

When you use await with reduce, the results get extremely messy.

// Reduce if we await getNumFruit
const reduceLoop = async _ => {
console.log(“Start”);
const sum = await fruitsToGet.reduce(async (sum, fruit) => {
const numFruit = await getNumFruit(fruit);
return sum + numFruit;
}, 0);
console.log(sum);
console.log(“End”);
};
“Start”;
“[object Promise]14”;
“End”;

What?! [object Promise]14?!

Dissecting this is interesting.

  • In the first iteration, sum is 0. numFruit is 27 (the resolved value from getNumFruit(‘apple’)). 0 + 27 is 27.
  • In the second iteration, sum is a promise. (Why? Because asynchronous functions always return promises!) numFruit is 0. A promise cannot be added to an object normally, so the JavaScript converts it to [object Promise] string. [object Promise] + 0 is [object Promise]0
  • In the third iteration, sum is also a promise. numFruit is 14. [object Promise] + 14 is [object Promise]14.

Mystery solved!

This means, you can use await in a reduce callback, but you have to remember to await the accumulator first!

const reduceLoop = async _ => {
console.log(“Start”);
const sum = await fruitsToGet.reduce(async (promisedSum, fruit) => {
const sum = await promisedSum;
const numFruit = await getNumFruit(fruit);
return sum + numFruit;
}, 0);
console.log(sum);
console.log(“End”);
};
“Start”;
“41”;
“End”;

But... as you can see from the gif, it takes pretty long to await everything. This happens because reduceLoop needs to wait for the promisedSum to be completed for each iteration.

There's a way to speed up the reduce loop. (I found out about this thanks to Tim Oxley. If you await getNumFruits() first before await promisedSum, the reduceLoop takes only one second to complete:

const reduceLoop = async _ => {
console.log(“Start”);
const sum = await fruitsToGet.reduce(async (promisedSum, fruit) => {
// Heavy-lifting comes first.
// This triggers all three getNumFruit promises before waiting for the next iteration of the loop.
const numFruit = await getNumFruit(fruit);
const sum = await promisedSum;
return sum + numFruit;
}, 0);
console.log(sum);
console.log(“End”);
};

This works because reduce can fire all three getNumFruit promises before waiting for the next iteration of the loop. However, this method is slightly confusing since you have to be careful of the order you await things.

The simplest (and most efficient way) to use await in reduce is to:

1. Use map to return an array promises

2. await the array of promises

3. reduce the resolved values

const reduceLoop = async _ => {
console.log(“Start”);
const promises = fruitsToGet.map(getNumFruit);
const numFruits = await Promise.all(promises);
const sum = numFruits.reduce((sum, fruit) => sum + fruit);
console.log(sum);
console.log(“End”);
};

This version is simple to read and understand, and takes one second to calculate the total number of fruits.

Key Takeaways

1. If you want to execute await calls in series, use a for-loop (or any loop without a callback).

2. Don't ever use await with forEach. Use a for-loop (or any loop without a callback) instead.

3. Don't await inside filter and reduce. Always await an array of promises with map, then filter or reduce accordingly.


This article was originally posted on my blog.
Sign up for my newsletter if you want more articles to help you become a better frontend developer.

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Zell Liew

Written by

Zell Liew

Frontend consultant; teaching frontend to self-taught devs at zellwk.com. Currently attempting to demystify Javascript at https://learnjavascript.today

freeCodeCamp.org

This is no longer updated. Go to https://freecodecamp.org/news instead

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