opentype.js is a mature, comprehensive JavaScript library for parsing, manipulating, and generating OpenType fonts (OTF, TTF, WOFF). It runs in both browser and Node.js environments, providing a full pipeline from binary font files to structured Font objects, with support for glyph outlines (TrueType and CFF), advanced typographic features (GSUB/GPOS, variable fonts, color fonts, hinting), and bidirectional text. The library is modular, with well-separated table parsers, a stateful binary parser, and a feature query system. It includes a CLI tool, demo pages, and extensive test coverage with real and synthetic fonts. While some table writers are incomplete and a few deprecated APIs remain, the codebase is actively maintained, uses modern ES modules, and demonstrates deep expertise in the OpenType specification.