no-await-in-loop
Disallow await inside of loops
Performing an operation on each element of an iterable is a common task. However, performing an
await as part of each operation is an indication that the program is not taking full advantage of
the parallelization benefits of async/await.
Usually, the code should be refactored to create all the promises at once, then get access to the
results using Promise.all(). Otherwise, each successive operation will not start until the
previous one has completed.
Concretely, the following function should be refactored as shown:
async function foo(things) {
const results = [];
for (const thing of things) {
// Bad: each loop iteration is delayed until the entire asynchronous operation completes
results.push(await bar(thing));
}
return baz(results);
}
async function foo(things) {
const results = [];
for (const thing of things) {
// Good: all asynchronous operations are immediately started.
results.push(bar(thing));
}
// Now that all the asynchronous operations are running, here we wait until they all complete.
return baz(await Promise.all(results));
}
Rule Details
This rule disallows the use of await within loop bodies.
Examples
Examples of correct code for this rule:
/*eslint no-await-in-loop: "error"*/
async function foo(things) {
const results = [];
for (const thing of things) {
// Good: all asynchronous operations are immediately started.
results.push(bar(thing));
}
// Now that all the asynchronous operations are running, here we wait until they all complete.
return baz(await Promise.all(results));
}
Examples of incorrect code for this rule:
/*eslint no-await-in-loop: "error"*/
async function foo(things) {
const results = [];
for (const thing of things) {
// Bad: each loop iteration is delayed until the entire asynchronous operation completes
results.push(await bar(thing));
}
return baz(results);
}
When Not To Use It
In many cases the iterations of a loop are not actually independent of each-other. For example, the
output of one iteration might be used as the input to another. Or, loops may be used to retry
asynchronous operations that were unsuccessful. Or, loops may be used to prevent your code from sending
an excessive amount of requests in parallel. In such cases it makes sense to use await within a
loop and it is recommended to disable the rule via a standard ESLint disable comment.
Version
This rule was introduced in ESLint v3.12.0.