This paper introduces lfo — for Low Frequency Operators — a graph-based Javascript (ES2015) API for online and offline processing (i.e. analysis and transformation) of data streams such as audio and motion sensor data. The library is open-source and entirely based on web standards. The project aims at creating an ecosystem consisting of platform-independent stream operator modules such as filters and extractors as well as platform-specific source and sink modules such as audio i/o, motion sensor inputs, and file access. The modular approach of the API allows for using the library in virtually any Javascript environment. A first set of operators as well as basic source and sink modules for web browsers and Node.js are included in the distribution of the library. The paper introduces the underlying concepts, describes the implementation of the API, and reports on benchmarks of a set of operators. It concludes with the presentation of a set of example applications.