> For the complete documentation index, see [llms.txt](https://practical-testing.gitbook.io/home/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://practical-testing.gitbook.io/home/test-automation/mutation-testing.md).

# Mutation Testing

### Overview

Mutation testing is a white-box technique to help improve code coverage. **In a nutshell, it is about injecting bugs into your own code and seeing if they get caught.**

More specifically, it is:

1. Changing production code and its various operators. Such changes are called <mark style="color:orange;">"mutants"</mark>. Examples include:
   1. Replacing `<` with `<=`&#x20;
   2. Flipping `==` to `!=`
   3. Replacing `return calculationResult;` with `return 0;`
2. Running existing tests.
3. Ensuring that at least one test failed (thus caught the change).&#x20;
   1. A failing test means the introduced mutant was <mark style="color:orange;">"killed"</mark>.
   2. If all tests pass, then the mutant <mark style="color:orange;">"survived"</mark>, and it means the test suite has gaps that should be addressed.

Mutation testing is a response to some inadequate [coverage types](/home/test-automation/code-coverage-types.md), such as Line Coverage.

Even Branch Coverage is insufficient when the tests or test data are poor, for example:

* Boundary values are not used
* Compound conditions (such as `(a && b || (c && d))` ) are weakly tested
* Unless the stricter [MC/DC coverage](https://practical-testing.gitbook.io/home/test-automation/code-coverage-types#modified-condition-decision-coverage-mcdc) is applied, there is no guarantee that the independent influence of each condition is verified
* Missing assertions in tests (test is green, coverage exists, but nothing is really checked)

To summarize:

| Coverage Practice          | Added Value from Mutation Testing |
| -------------------------- | --------------------------------- |
| Line coverage              | Very high                         |
| Branch / Decision coverage | Moderate to high                  |
| Branch + MC/DC coverage    | Moderate to low                   |

### Popular tools

Some of the currently popular tools. The list may change over time.

* **JavaScript / TypeScript**: Stryker, Mutant
* **Python**: mutmut, MutPy
* **Java**: PIT (Pitest)
* **C# / .NET**: Stryker.NET
* **Go**: Go-Mutesting