qs is a JavaScript library for parsing and stringifying URL query strings with support for nested objects, arrays, and custom formatting. It is designed as a drop-in replacement for Node.js's built-in querystring module but with enhanced security features, including protection against prototype pollution, configurable depth limits, and parameter/array size limits. The library exposes three main functions: `parse`, `stringify`, and `formats`, all accessible from the single entry point `lib/index.js`. Internally, it uses a side-channel pattern for cycle detection and overflow tracking, and implements a chunked UTF-8 encoder to handle surrogate pairs safely. The codebase is well-tested with an extensive suite of edge-case tests, and it is distributed as an npm package with browser builds via Browserify. Architecturally, qs is a focused, single-purpose library with a clean separation of concerns: parsing logic in `lib/parse.js`, serialization in `lib/stringify.js`, shared utilities in `lib/utils.js`, and formatting constants in `lib/formats.js`. It follows a functional style with option normalization functions that validate and fill defaults, making the API robust against misuse. The library has a long history (over a decade) and is widely adopted (nearly 9000 stars), but its maintenance shows signs of high release churn and legacy support burdens, including backporting fixes to older Node.js versions (>=0.6) and maintaining outdated packaging artifacts like `bower.json` and `component.json`. Despite these legacy concerns, the core code is well-crafted, security-conscious, and actively maintained with frequent releases and responsive issue triage.