What is Mutation Testing?

Stryker's documentation landing page explains mutation testing — the practice of injecting small deliberate bugs ("mutants") into production code and checking whether the unit tests catch them. The central claim: line coverage measures whether code was executed, not whether a test actually asserted anything about it, so a high-coverage suite can still be toothless.

The page distinguishes the two outcomes that matter. A mutant is killed when at least one test fails against the mutated code; it survived when every test still passes — evidence that no test actually pins down that behaviour. The ratio of killed to total mutants becomes a measure of test-suite effectiveness, which the page argues coverage alone cannot provide.

Stryker itself is introduced as one tool that "uses one design mentality to implement mutation testing on three platforms" (JavaScript/TypeScript, C#, Scala), promising it is easy to use and fast to run. Its one concrete design claim: it mutates only source code, never test code, so that a mutant failing is always a signal about test quality rather than an artifact of the harness.

A worked example walks through isUserOldEnough(user) { return user.age >= 18; }, showing two mutators (BinaryOperator, which flips >= to >, and RemoveConditionals, which replaces the whole expression with true), then shows the clear-text reporter output distinguishing the killed mutant from the survivor — the survivor flagged as "probably a test missing that explicitly tests for age lower than 18."