Ajv is a popular JavaScript library used to validate a data object against a structure defined using a JSON Schema. Ajv is fast and flexible for validating complex data types. It supports a wide range of schema features, including custom keywords and formats. It's used by many open-source JavaScript applications and libraries. Our teams have used Ajv for implementing consumer-driven contract testing in CI workflows, and, along with other tools for generating mock data with the JSON Schema, it's very powerful. In the TypeScript world, Zod is a popular alternative that has a declarative API for defining the schema and validating the data.