es6-shim
ECMAScript 6 compatibility shims for legacy JavaScript engines, providing modern ES6 features in older environments.
What is es6-shim?
ES6 Shim is a JavaScript compatibility library that provides shims for ECMAScript 6 features in legacy JavaScript engines. It solves the problem of inconsistent or missing ES6 support across different browsers and Node.js versions by implementing modern JavaScript APIs like Promises, Maps, Sets, and new array/string methods in environments where they aren't natively available.
JavaScript developers working with legacy browsers or older Node.js versions who need to use ES6 features while maintaining cross-environment compatibility. It's particularly valuable for teams supporting Internet Explorer or other browsers with incomplete ES6 implementations.
Developers choose ES6 Shim because it provides comprehensive, spec-compliant implementations of ES6 features with a focus on correctness and safety. Unlike some polyfills, it avoids incorrect implementations and offers modular standalone shims, making it a reliable choice for production applications that need consistent ES6 behavior across all environments.
Overview
ECMAScript 6 compatibility shims for legacy JS engines
Use Cases
Best For
- Adding ES6 support to legacy Internet Explorer browsers
- Using modern JavaScript features in older Node.js versions
- Ensuring consistent Promise implementation across different environments
- Implementing Map and Set collections in browsers without native support
- Using ES6 string methods like startsWith() and endsWith() in legacy codebases
- Adding Object.assign() and other ES6 object utilities to older projects
Not Ideal For
- Applications targeting only modern browsers and Node.js versions with native ES6 support
- Projects requiring ES2016 or later JavaScript features beyond ES6
- Teams using build tools like Webpack and wanting to tree-shake polyfills for minimal bundle size
- Systems with critical performance requirements where any polyfill overhead is prohibitive
Pros & Cons
Pros
Implements a wide array of ES6 features including Map, Set, Promise, and new string/array methods, as detailed in the 'Safe shims' section of the README.
Prioritizes correctness over polyfilling everything, avoiding incorrect shims like WeakMap and ensuring behavior matches the ES6 specification, as stated in the philosophy.
Offers individual npm packages for each feature, allowing developers to include only necessary shims, with many 'a standalone shim is also available' links in the README.
Enables subclassing of Map, Set, and Promise with proper prototype chain setup, with code examples provided in the README section on subclassing.
Cons
Does not include a WeakMap implementation because it's impossible to implement correctly in pure JavaScript, limiting completeness for ES6 features as admitted in the README.
String.prototype.normalize requires installing the separate 'unorm' package, adding complexity and an extra dependency, as noted in the README section on normalize.
Strongly recommends including es5-shim for broken ES5 implementations, increasing setup burden and bundle size, as emphasized in the installation instructions.
Polyfilling can introduce performance penalties compared to native implementations, especially for heavy features like Promises or collections, which may impact resource-constrained applications.
Frequently Asked Questions
Related Projects
core-jsStandard Library
es6-promiseA polyfill for ES6-style Promises
ES6 shimJavaScript Polyfills, Shims and More
Object.assignES2015 Object.assign() ponyfill
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