Who tests your unit tests? Even if a line of code is covered by a test, you're not guaranteed the test verifies anything. Enter mutation testing: the practice of inserting "mutant" bugs into your code and checking whether any tests catch them.

This hands-on talk will walk you through real-world code issues detectable with mutation testing, how mutation testing catches them, and how to get started with it locally.

In particular, we'll cover:
* Where unit testing coverage does or does not act as an effective measure of behavioral coverage
* Common types of bugs and developer mishaps
* How mutation testing works at a theoretical level (AST analysis, mutators, and strategies for efficient re-testing)
* Running mutation testing on an example TypeScript repository
* Integrating mutation testing both into technical needs (i.e. CI/builds) and interpersonal (e.g. team culture)
* Creating our own custom mutators for business-specific use cases
* Where mutation testing does not catch code slipups and/or can be augmented with other forms of static or dynamic analysis

We'll view demos of TypeScript code with StrykerJS but the lessons learned will be applicable across any stack.