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Why Is My Jest Test Suite So Slow?
The simple mistake undermining Jest’s performance
Our team is a couple of months into developing a new application, and our suite of unit 240 tests takes 46 seconds to run. That duration is not excessive yet, but it’s increasing in proportion to the number of tests. In a couple of months, it’ll take a couple of minutes to run our tests.
We were surprised by this, as Jest is known for its fast performance. However, while Jest reported that each test only took 40ms, the overall run time for each test was closer to 6 seconds.
The integration tests for one of our legacy applications fare even worse, taking around 35 seconds for a single test. This time puts it over the duration where the mind starts to wander, and it’s hard to focus on developing the tests. With each actual test only taking about a second, where is all the extra time going?
Over the past couple of weeks, I’ve fallen down a bit of a rabbit hole trying to figure out why our test suite is so slow. Unfortunately, there are a lot of ideas out there to sort through, and few of them had any impact. Further, there doesn’t even seem to be much of a consensus on how fast our tests should be.
The outcome of this investigation was a reduction of the duration of our unit tests from 46 to 13 seconds. Our integration tests saw a similar improvement, with their duration falling from 35 to 15 seconds. Our pipelines saw even more significant improvements, which I cover in this separate article.
In this article, I want to share the improvements that made the biggest differences, as well as look at some of the possible misconfigurations and misusages of Jest undermining its performance.
While the following example appears simple and like it should run really fast, it hides a surprising but very common configuration that will delay our tests significantly.
// TestComponent.tsx
import {Button} from "@mui/material";
export const TestComponent = () => {
return <Button>Hello World!</Button>;
}
// ComponentB.test.tsx
import React from 'react'…
