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Jest

A collection of 16 posts

#jest #testing #javascript #node

Jest set, clear and reset mock/spy/stub implementation

Between test runs we need mocked/spied on imports and functions to be reset so that assertions don’t fail due to stale calls (from a previous test). This is a way to mitigate what little statefulness is in the system. In unit tests of complex systems, it’s not always ...

  • Hugo Di Francesco
    Author
7 min read
#jest #testing #javascript #node

Jest .fn() and .spyOn() spy/stub/mock assertion reference

Where other JavaScript testing libraries would lean on a specific stub/spy library like Sinon - Standalone test spies, stubs and mocks for JavaScript. Works with any unit testing framework., Jest comes with stubs, mocks and spies out of the box. This post looks at how to instanti ...

  • Hugo Di Francesco
    Author
6 min read
#jest #node #javascript #testing

Jest Array/Object partial match with objectContaining and arrayContaining

It’s possible to do partial matches on Arrays and Objects in Jest using expect.objectContaining and expect.arrayContaining. expect has some powerful matcher methods to do things like the above partial matches. Using Jest at an advanced level means using tools like these to ...

  • Hugo Di Francesco
    Author
5 min read
#javascript #jest #node #testing

Jest explicitly or arbitrarily force fail() a test

When testing code with Jest, it can sometimes be useful to fail a test arbitrarily. This post goes through a few scenarios where that might be useful and how to fail a Jest test explicitly/in a forced manner. It also presents more idiomatic Jest patterns that could be used interc ...

  • Hugo Di Francesco
    Author
10 min read
#javascript #jest #node #es6 #testing

Jest Full and Partial Mock/Spy of CommonJS and ES6 Module Imports

JavaScript import/require module testing do’s and don’ts with Jest The example repository is available at github.com/HugoDF/mock-spy-module-import. This post goes through how to achieve different types of module mocking scenarios with Jest. From simple Import interc ...

  • Hugo Di Francesco
    Author
13 min read
#jest #javascript #node #testing

How to run, ignore or skip Jest tests, suites and files

When debugging or writing units tests with Jest it can be useful to specify a single file or test to run or to exclude a specific failing test for the Jest run. This post goes how to skip and exclude a single test, a whole Jest test suite and a whole Jest test file using the CLI ...

  • Hugo Di Francesco
    Author
9 min read
#jest #javascript #node #testing

How to run Jest tests sequentially

By default Jest runs tests in parallel with a “a worker pool of child processes that run tests” (Jest CLI docs). As per the Jest documentation, running tests in sequence (serially), can be helpful for debugging purposes and for test run performance reasons. ...

  • Hugo Di Francesco
    Author
2 min read
#javascript #jest #testing #node

Jest ignore or exclude file/function/statement from test coverage

In computer science, test coverage is a measure used to describe the degree to which the source code of a program is executed when a particular test suite runs. Code coverage - Wikipedia Code coverage is usually used as a quality metric for software eg. “Our code has to have 80 ...

  • Hugo Di Francesco
    Author
4 min read
#javascript #jest #node #Testing #date

Mocking/stubbing the current Date in Jest tests

There are situations where new Date() or Date.now is used in application code. That code needs to be tested, and it’s always a struggle to remember how to mock/stub or spy on Date.now/new Date with Jest. This post goes through multiple approaches to mocking, stubbing and spying o ...

  • Hugo Di Francesco
    Author
3 min read
#javascript #node #jest #testing #MongoDB

Avoiding recursion pitfalls: MongoDB nested query clause addition and removal

A case-study on where recursion can be useful for enterprise Node.js applications and how to avoid its common pitfalls like RangeError: Maximum call stack size exceeded. The full repository for this post is on GitHub: github.com/HugoDF/mongo-query-clause-modification We’ll ...

  • Hugo Di Francesco
    Author
7 min read
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