k6

k6.io

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An open-source load testing tool that uses JavaScript scripts to define virtual user scenarios, with output compatible with Prometheus and Grafana and support for HTTP, WebSockets, and gRPC.

Why it is useful

Tests are just JavaScript files you can version-control and review like application code, with a clean API and built-in thresholds that fail the build if SLOs are missed. The same script runs locally for quick sanity checks and scales to thousands of virtual users in the cloud. Free and open source with a paid cloud option — the developer-friendliest load testing tool available.