FIT Parser

FIT Parser is the MIT-licensed JavaScript and TypeScript library published on npm as fit-file-parser. It parses and encodes FIT activity files, supports developer-defined data and raw messages, and provides browser and Node.js inputs with ESM and CommonJS builds.

It is a reusable dependency for applications importing activity data from compatible sports devices. Sports Lib depends on it, and Quantified Self is a downstream application. Independent repository manifests also declare it as a dependency. The npm listing observed on 4 November 2026 reported approximately 49,000 weekly downloads and 12 dependent packages; downloads are not distinct users. Package: https://www.npmjs.com/package/fit-file-parser

Funding would support issue triage, reproducible compatibility fixes, activity-file regression fixtures, release maintenance and migration guidance. Existing validation tests, compatibility checks and documentation are a foundation to extend, rather than replace.