I'm a big fan of mutation testing and I've converted a few other devs I've worked with to it. I use a tool called Stryker Mutator in C# and JavaScript/TypeScrip

I'm a big fan of mutation testing and I've converted a few other devs I've worked with to it. I use a tool called Stryker Mutator in C# and JavaScript/TypeScript to automate the bug-injection. It adds a little bit of overhead - a small-ish TS project where our normal test suite runs in 2 minutes or so takes about 20-25 with Stryker - but it has definitely found things that our tests weren't really covering as well as they should have before.