MongoDB has developed a JavaScript fuzzer that is its most prolific bug-finding tool, responsible for finding almost 200 bugs over the course of two release cycles. The fuzzer hybridizes smart and dumb fuzzing techniques to achieve greater coverage than either method alone could accomplish. It uses an abstract syntax tree (AST) to model valid input surfaces and generates tests by randomly mutating these trees, while also using a seed list of edge cases to produce new test inputs that may not be caught by traditional testing methods. The hybrid approach combines the strengths of both smart and dumb fuzzing, allowing it to find bugs that would otherwise go undetected.