es5shim

es5shim, commonly written as es5-shim, is an open-source JavaScript library that provides polyfills for ECMAScript 5 features in environments that do not implement them natively. By adding missing methods and properties to built-in objects, it enables developers to use ES5-style code while maintaining compatibility with older browsers and runtimes.

The library covers a broad set of ES5 functionality. Typical shims include Array methods such as forEach, map, reduce, filter, every, some, indexOf, and lastIndexOf; Object methods like create, defineProperty, defineProperties, getOwnPropertyDescriptor, getOwnPropertyNames, keys, and getPrototypeOf; and utilities such as Object.freeze, Object.seal, Object.preventExtensions, as well as Function.prototype.bind and Date.now. It also provides JSON support in environments lacking a native JSON object. es5shim aims to mimic ES5 semantics where feasible, including handling of non-enumerable properties and property descriptors, by augmenting the global objects when native implementations are missing or incomplete.

Limitations exist: in some engines, certain ES5 details cannot be perfectly polyfilled, particularly regarding property descriptors on non-DOM objects or engine-specific bugs. The project provides best-effort shims and practical workarounds to improve compatibility, not guarantees of exact parity with all ES5 behavior.

Context and usage: es5shim is one option among polyfills used to maintain legacy web codebases while gradually adopting newer JavaScript features. Modern environments typically include native ES5 support, and contemporary polyfills such as core-js or es6-shim address later standards. The project is maintained as an open-source effort with community contributions.