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Should you use jest as a testing library?
Learn why it breaks the instanceof operator
jest
, the popular testing framework created by Facebook with over 50 million downloads per month, causes many problems for backend developers.
In this article, I try to recap the jest
's cross and delight and how to deal with it and what is causing it.
How does jest work?
jest
is a JavaScript Testing Framework with many features, including isolated
written on its website.
The isolation feature's target performance:
Tests are parallelized by running them in their own processes to maximize performance.
The jest
's isolation comes from its architecture that uses the node:vm
core module under the hood.
The vm
module lets jest
run every test file into a sandbox with its own temporary context.
The context includes all the global
classes such Array
, Error
, Date
and many others - the describe
and the it
test function for example.
(Here the jest source code that does the trick)
Since jest
overwrites some of those components to provide you fancy features like mocks, fake clocks and a fast test execution,
it must set all the vm
's global
data to set up the context where the test's source code will be executed.
Unfortunately, when the Node.js core modules create a new instance of a global
class,
they will not use the vm
's context, but they fall back to the native implementation.
This means that the instanceof
operator will not work
as expected, and it will generate false negatives!
You can get a quick example of this problem in the following test.js
snippet file:
const { parseArgs } = require('util')
test('Array is not an Array', async () => {
const { values } = parseArgs({
args: ['--bar', 'a', '--bar', 'b'],
options: {
bar: {
type: 'string',
multiple: true
}
}
})
expect(values.bar).toEqual(['a', 'b'])
expect(values.bar).toBeInstanceOf(Array) // โ it will fail
})
By running the above test with the command jest test.js
(with the default jest
configuration)
it will fail with the stange error:
โ Array is not an Array
expect(received).toBeInstanceOf(expected)
Expected constructor: Array
Received constructor: Array
27 | })
28 | expect(values.bar).toEqual(['a', 'b'])
> 29 | expect(values.bar).toBeInstanceOf(Array)
This may seem a minor problem, you need just to avoid using the instanceof
operator, but it is not.
The bigger problem is that the instanceof
operator could be used by your dependencies tree to perform
some checks, and those conditions will fail.
For example, Fastify removed the instanceof
operator
from its codebase because it was causing problems for those developers that rely on jest
as a testing framework.
How to fix it?
It depends ๐
You can't out of the box.
There is an open issue on the Node.js repository
to let the node:vm
module to use the vm
's context, but it is still open.
It seems that the Node.js core team is interested in fixing this problem by implementing the new ShadowRealm spec, and I think we will make some progress during 2023.
If you can't wait, there is a very quick solution by using the jest-environment-node-single-context
custom test enviroment.
If you install this module and you run the previous code with the command:
jest --testEnvironment jest-environment-node-single-context test.js
Now, all will work as expected:
PASS ./test.js
โ Array is not an Array (5 ms)
โ ๏ธ Note that now the test is working because the jest
isolation feature is totally disabled by running all the tests
in the same context.
Another working solution is to use a new jest
runner: jest-light-runner
.
It spin up a Node.js worker thread for each test file giving you the same isolation feature of jest
but without the vm
's context.
Note that not all the jest
features are supported by this runner, but all the most used features are working!
So, after installing it, you can verify that the tests are green by running the command:
jest --runner jest-light-runner test.js
Summary
jest
is a great testing framework, and it works well for frontend applications,
but you could face some issues if your dependencies rely on instanceof
.
We discussed about how to fix this problem in different ways:
- Adopt the
jest-environment-node-single-context
custom test environment and its limitations - Use the
jest-light-runner
runner to get the same isolation feature ofjest
but with a subset of its features - Open an issue to the repository of the module that you are using and ask to remove the
instanceof
operator, that is still a good practice - Choose another test framework. I personally prefer
node-tap
The jest
architecture makes sense for frontend applications that run in the browser and has a different global
context, but it does not suit the case for Node.js applications.
I don't like the concept of using a different global
context in my test and production environment.
Now jump into this article source code to try the code snippets I wrote to verify my findings.
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