MiniPoly: Automatic Extraction of Efficient JavaScript Polyfill from Language Specification

Abstract

A polyfill is a JavaScript implementation of built-in APIs for older execution environments lacking native support. Because developers rely on them to run modern features everywhere, strict conformity to the official language specification (ECMA-262) is crucial. However, manual implementation remains the industry standard; this process is labor-intensive, inherently error-prone, and frequently leads to spec-miscompliance bugs in production. To address this, we present MiniPoly, the first compilation framework to automatically extract correct and optimized polyfills directly from the ECMA-262 specification. To ensure correct generation, MiniPoly employs a static analysis that resolves the specification’s internal control-flow representations (i.e., completion records) and translates them into equivalent JavaScript constructs. Furthermore, it utilizes rewrite rules to apply user-defined optimization patterns, replacing algorithmic bottlenecks in the specification with highly efficient implementations. Evaluation shows that MiniPoly successfully generates polyfills for 89 built-in APIs in ECMAScript 2025 (ES2025) using 17 user-defined rewrite rules. It produces a more spec-compliant polyfill than production-standard libraries like core-js and es-shims with a competitive performance profile. Additionally, we demonstrate that MiniPoly can generate polyfills even for the next draft release of ECMA-262 and early-stage proposals, highlighting its potential to future-proof development.