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Bun's Test Runner: The Future of JavaScript Testing?

Nov 3rd 2023 7 min read

Are you familiar with Bun, the emerging sensation in the JavaScript ecosystem? This fast, all-in-one JavaScript runtime and toolkit is specifically engineered to serve as a seamless replacement for Node.js. It is built from scratch using the Zig programming language, which gives it several advantages over Node.js, such as improved performance, reliability, and security.

As a comprehensive toolkit, Bun includes a built-in test runner packed with features. Could this be the next big thing in automation testing? Let's see.

Bun's test runner

Bun's test runner is a fast, built-in, Jest-compatible test runner that is designed to be a viable alternative to other popular test runners. It is written in Zig, a modern systems programming language that is known for its performance and reliability.

Bun's test runner supports all of the features that you would expect from a modern test runner, including:

In addition to these features, Bun's test runner also offers a number of advantages over other test runners:

Other popular test runners

In the vast landscape of JavaScript testing frameworks, let's take a closer look at some of Bun's test runner's notable counterparts.


Jest is a popular test runner known for its ease of use and comprehensive set of features. It is built on top of Jasmine and offers numerous features that simplify test writing and execution, including automatic mocking, snapshot testing, and watch mode. Jest is an excellent choice for teams seeking an easy-to-start-with test runner with a wide range of features.


Mocha is a flexible and extensible test runner renowned for its speed and customizable nature. Unlike Jest, Mocha doesn't offer as many built-in features, but it excels in flexibility and can be tailored to meet a team's specific requirements. Mocha is an excellent choice for teams seeking a fast test runner that can be adapted to their individual needs.


Jasmine is a behavior-driven development (BDD) test framework well-known for its expressiveness and readability. It is an excellent choice for teams seeking a test runner that is easy to read and promotes the use of BDD.

Comparison of features

Now that we have a firm grasp of Bun's test runner and its popular counterparts, let's explore a thorough comparison of their key features. This exploration will provide us with a more profound understanding of their strengths and weaknesses, ultimately helping us determine whether Bun's test runner has the potential to establish itself as a dominant force in the realm of JavaScript testing.


The initial aspect we aim to compare is, of course, Bun's most significant selling point: speed. We generated test suites for each of the runners/frameworks mentioned above, incorporating identical test cases. A cumulative total of 5000 test cases were executed 10 times for each runner. The following are the average execution times obtained:

Runner/framework Average execution time (ms)
Bun 505
Jasmine 544
Jest 5142
Mocha 570

In the realm of relatively simple automation tests, Bun outpaced Jasmine by approximately 7.16% in execution speed, clocking in at 505 milliseconds, demonstrating its faster performance in this specific testing scenario. Nice!


Reliability is an important factor to consider when choosing a test runner. A reliable test runner can help us to:

Bun's test runner stands out for its exceptional reliability, leveraging sandboxing, garbage collection, and proficient error handling mechanisms.

In the realm of error handling, Jasmine, Mocha, and Jest, the three prominent JavaScript testing frameworks, each possess their distinct error handling approaches. The effectiveness of these methods varies based on developers' preferences and project requirements.

Jasmine relies on its assertions and expectations for basic error handling. It captures and presents clear messages in the event of assertion failures, elucidating the nature of the error.

Mocha offers flexibility by allowing the use of diverse assertion libraries, like Chai or expect.js, providing developers with multiple options for customized error handling. When an assertion fails, Mocha reports detailed information via the employed assertion library.

Jest, known for its user-friendly features, leads the pack with superior error handling capabilities. In the event of a test failure, Jest offers detailed and structured insights, allowing a clear understanding of the encountered issues.

Bun's exceptional performance in this domain stems from its strategic use of garbage collection, ensuring efficient memory management, and sandboxing, which isolates tests from one another.

While none of the major JavaScript testing frameworks, namely Jasmine, Mocha, or Jest, directly manages garbage collection, the responsibility typically falls on the underlying JavaScript engine. Additionally, none of these frameworks integrates built-in sandboxing features.

To create controlled testing environments, developers often leverage sandboxing techniques such as mocking, stubbing, or dependency injection. However, Jest stands apart by offering specific functionalities like jest.mock() for module mocking, jest.fn() to create mock functions, and jest.spyOn() to generate method spies, enabling the creation of isolated testing environments that remain independent of external dependencies or systems.

In conclusion, while each framework offers unique error handling strategies, Bun's emphasis on reliable error handling, aided by strategic environment isolation techniques, sets it apart in the realm of JavaScript testing.

Ease of use

Bun's test runner is tailored for ease of use, especially for newcomers in the testing landscape. Its simple and intuitive API, accompanied by several user-friendly features like automatic mocking, snapshot testing, watch mode, and script pre-loading, simplifies test writing and execution. Additionally, the built-in documentation tool within Bun aids developers in learning the ropes of using this tool effectively.

Jasmine is also acclaimed for its expressiveness and readability. Its user-friendly attributes include a straightforward syntax, support for asynchronous testing, and a built-in test runner. Jasmine benefits from an extensive and vibrant community, ensuring ample resources for developers to kickstart their journey and troubleshoot any encountered issues.

Mocha stands out for its flexibility, speed, and customizable nature. With a simple and adaptable API, support for asynchronous testing, and a built-in test runner, Mocha caters to user convenience. Similar to Jasmine, Mocha enjoys a substantial and active community, offering comprehensive support for developers, from beginners to seasoned professionals.

Jest, renowned for its user-friendly approach and rich feature set, builds upon Jasmine's foundation. Offering automatic mocking, snapshot testing, watch mode, script pre-loading, and its inbuilt test runner, Jest delivers a comprehensive testing experience. Supported by a thriving community, developers have access to abundant resources for initiating their Jest journey and resolving potential challenges.

In conclusion, all the discussed testing frameworks offer user-friendly features and functionalities, making them relatively straightforward for developers to use.


Is Bun the best test runner out there now? Hard to say. It has many great built-in features that other frameworks require libraries for. It outperformed other frameworks in the custom speed benchmark, but there have been instances where the differences were barely noticeable.

We must remember that Bun is not a test framework but a toolkit with many more under-the-hood features that we haven't talked about. It is still under development, and I'm sure there will be more great features to come. In my opinion, Bun is heading in the right direction. What are your thoughts?