JavaScript IE/Edge Compatibility

As JavaScript evolved through ECMAScript versions (ES6/ES2015 onwards), browser support has varied significantly. Microsoft’s Internet Explorer (IE) has historically lagged behind, while Microsoft Edge has progressively aligned with modern standards. Understanding compatibility is critical for developers maintaining legacy applications or targeting enterprise environments.

Internet Explorer (IE) Compatibility

Last Major Version: IE11 (2013)

JavaScript Engine: Chakra (pre-EdgeHTML), primarily ES5 with limited ES6 support.

Status: Fully deprecated for general use as of June 15, 2022. Extended security updates continue for enterprise users until 2029.

Market Share: ~0.5% globally (mostly legacy enterprise systems).

Key Limitations for ES6+ Features:

• let / const → Not supported, Babel transpilation required.
• Arrow functions → Syntax errors in IE11.
• Promises → Not native; requires polyfills (core-js or es6-promise).
• async/await → Unsupported; must transpile to generator functions or Promise chains.
• Classes & custom errors → Syntax not supported; requires transpilation.
• Optional chaining (?.) and Error.cause → Not available; Babel polyfill required.

Error Handling: Only basic try-catch-finally and throw work natively. Modern async error handling via Promises or async/await is unsupported without polyfills.

Enterprise Workarounds:

• Polyfills: Add missing JS features for IE11.
• Transpilers: Convert modern JS (ES6+) to ES5 using Babel.
• IE Mode in Edge: Run legacy web applications using Microsoft Edge’s IE Mode.

Microsoft Edge Compatibility

Legacy Edge (EdgeHTML, 2015–2019):

• Partial ES6 support.
• Limited Promises and async capabilities.

Chromium-based Edge (2020+):

• Full ES6+ support, including Promises, async/await, optional chaining, and Error.cause.
• Supports all modern ECMAScript features on par with Chrome.
• Default Windows browser; regularly updated every ~6 weeks.

Advantages: Modern Edge eliminates most cross-browser compatibility issues, enabling full use of ES6+ features and advanced error-handling mechanisms.

Key ES6+ JavaScript Features and Compatibility

Feature	Description	IE11	Legacy Edge	Chromium Edge
let / const	Block-scoped variables	None	Partial	Full
Arrow Functions	Concise function syntax	None	Partial	Full
Classes / Custom Errors	ES6 class syntax for inheritance and custom errors	None	Partial	Full
Promises	Async error handling with .catch()	Polyfill	Full	Full
async/await	Synchronous-like async code	Polyfill via transpile	Full	Full
Optional Chaining (?.)	Safe property access	None	None	Full
Error.cause	Nested error tracking	None	None	Full
Promise.finally	Cleanup after async operations	Polyfill	Partial	Full

Sources: MDN, CanIUse, Microsoft Docs, Stack Overflow, LambdaTest.

Internet Explorer Retirement Timeline

Microsoft has officially ended support for Internet Explorer, gradually phasing it out across services:

• August 17, 2020: IE retirement announcement.
• November 30, 2020: MS Teams ended IE11 support.
• December 31, 2020: Azure DevOps Services stopped supporting IE11.
• March 31, 2021: Azure Portal ends support.
• August 17, 2021: Office 365, OneDrive, Outlook ended IE11 support.
• June 15, 2022: All general support for IE11 officially ended.

Earlier Announcements:

• Windows 11 disables IE; replaced by Edge.
• Windows 10 “replaces” IE with Edge.
• Edge Legacy (EdgeHTML/Chakra) support ended March 9, 2021; replaced by Chromium-based Edge.

Legacy Use Cases:

• Some legacy systems and old Windows OS versions still include IE11.
• Older enterprise applications rely on ActiveX components or AJAX-based IE-specific APIs.

Solution: Microsoft Edge IE Mode allows running legacy web apps in a modern browser while maintaining compatibility.