Asynchronous Bugs in JavaScript

When your program runs synchronously, in a single go, there are no state changes happening except those that the program itself makes. For asyn­chronous programs this is different—they may have gaps in their execution during which other code can run.

Let’s look at an example. One of the hobbies of our crows is to count the number of chicks that hatch throughout the village every year. Nests store this count in their storage bulbs. The following code tries to enumer­ate the counts from all the nests for a given year:

function anyStorage(nest, source, name) {

if (source == return storage(nest, name);

async function chicks(nest, year) {

let list = “”;

await Promise.all(network(nest).map(async name => {

list += ‘${name}: ${

await anyStorage(nest, name, ‘chicks in ${year}’) }\n’;


return list;


The async name => part shows that arrow functions can also be made async by putting the word async in front of them.

The code doesn’t immediately look suspicious … it maps the async arrow function over the set of nests, creating an array of promises, and then uses Promise.all to wait for all of these before returning the list they build up.

But it is seriously broken. It’ll always return only a single line of output, listing the nest that was slowest to respond.

Can you work out why?

The problem lies in the += operator, which takes the current value of list at the time where the statement starts executing and then, when the await finishes, sets the list binding to be that value plus the added string.

But between the time where the statement starts executing and the time where it finishes there’s an asynchronous gap. The map expression runs before anything has been added to the list, so each of the += operators starts from an empty string and ends up, when its storage retrieval finishes, setting list to a single-line list—the result of adding its line to the empty string.

This could have easily been avoided by returning the lines from the mapped promises and calling join on the result of Promise.all, instead of building up the list by changing a binding. As usual, computing new values is less error-prone than changing existing values.

async function chicks(nest, year) {

let lines = network(nest).map(async name => {

return name + “: ” +

await anyStorage(nest, name, ‘chicks in ${year}’);


return (await Promise.all(lines)).join(“\n”);


Mistakes like this are easy to make, especially when using await, and you should be aware of where the gaps in your code occur. An advantage of JavaScript’s explicit asynchronicity (whether through callbacks, promises, or await) is that spotting these gaps is relatively easy.

Source: Haverbeke Marijn (2018), Eloquent JavaScript: A Modern Introduction to Programming, No Starch Press; 3rd edition.

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