What is Karate's embedded JavaScript engine, karate-js?

karate-js is Karate's own, purpose-built JavaScript engine, written from scratch in Java specifically for Karate's needs, replacing the previously used GraalJS engine (which itself had replaced the now-deprecated Nashorn engine in earlier Karate versions).

The motivation for building a custom engine rather than continuing with GraalJS was concurrency: GraalJS doesn't support using the same JS context safely across multiple threads, which conflicts directly with Karate's emphasis on running large test suites in parallel. karate-js was designed from the ground up to be thread-safe for concurrent execution, support modern ES6+ syntax, and integrate with Java without requiring a GraalVM-specific runtime, all while keeping startup time fast.

For everyday test authoring this change is mostly invisible: JavaScript expressions inside feature files, def functions, and karate-config.js continue to work the same way; the engine swap is primarily a compatibility and performance concern under the hood rather than something that changes how tests are written.