// AbortController polyfill (basic implementation)(function() { if ('AbortController' in window) return; function AbortSignal() { this.aborted = false; this.onabort = null; this._listeners = []; } AbortSignal.prototype.addEventListener = function(type, listener) { if (type === 'abort') { this._listeners.push(listener); } }; AbortSignal.prototype.removeEventListener = function(type, listener) { if (type === 'abort') { var index = this._listeners.indexOf(listener); if (index !== -1) { this._listeners.splice(index, 1); } } }; AbortSignal.prototype.dispatchEvent = function() { if (this.onabort) this.onabort(); this._listeners.forEach(function(listener) { listener(); }); }; function AbortController() { this.signal = new AbortSignal(); } AbortController.prototype.abort = function() { if (this.signal.aborted) return; this.signal.aborted = true; this.signal.dispatchEvent(); }; window.AbortController = AbortController; window.AbortSignal = AbortSignal;})();// Usage with fetchvar controller = new AbortController();var signal = controller.signal;fetch('/api/data', { signal: signal }) .then(function(response) { return response.json(); }) .catch(function(error) { if (error.name === 'AbortError') { console.log('Fetch aborted'); } });// Cancel request after 5 secondssetTimeout(function() { controller.abort();}, 5000);
Note: For production use, install whatwg-fetch and
abortcontroller-polyfill packages for full spec compliance. The examples above are simplified for
understanding.
// FormData polyfill for missing methods (IE11)(function() { if ('FormData' in window) { var originalFormData = window.FormData; // Add get method if missing if (!FormData.prototype.get) { FormData.prototype.get = function(name) { var entries = this.getAll(name); return entries.length ? entries[0] : null; }; } // Add getAll method if missing if (!FormData.prototype.getAll) { FormData.prototype.getAll = function(name) { // Fallback: cannot iterate in IE, return empty array return []; }; } // Add has method if missing if (!FormData.prototype.has) { FormData.prototype.has = function(name) { return this.get(name) !== null; }; } // Add set method if missing if (!FormData.prototype.set) { FormData.prototype.set = function(name, value, filename) { this.delete(name); this.append(name, value, filename); }; } // Add delete method if missing if (!FormData.prototype.delete) { FormData.prototype.delete = function(name) { // Cannot be implemented in IE }; } // Add entries iterator if missing if (!FormData.prototype.entries) { FormData.prototype.entries = function() { throw new Error('FormData.entries() not supported in this browser'); }; } }})();// Usagevar formData = new FormData();formData.append('username', 'john_doe');formData.append('email', 'john@example.com');formData.append('avatar', fileInput.files[0]);// Send via fetchfetch('/api/profile', { method: 'POST', body: formData});
Example: File constructor polyfill
// File constructor polyfill(function() { try { new File([], ''); return; // Native support } catch(e) {} // Polyfill File constructor window.File = function(bits, name, options) { options = options || {}; var blob = new Blob(bits, options); blob.name = name; blob.lastModified = options.lastModified || Date.now(); blob.lastModifiedDate = new Date(blob.lastModified); // Make it look like a File Object.defineProperty(blob, 'constructor', { value: File }); return blob; }; File.prototype = Object.create(Blob.prototype); File.prototype.constructor = File;})();// Usagevar file = new File(['Hello, World!'], 'hello.txt', { type: 'text/plain', lastModified: Date.now()});console.log(file.name); // 'hello.txt'console.log(file.size); // 13
Warning: FormData iteration methods (entries(), keys(),
values()) cannot be fully polyfilled in IE11. Use form-data polyfill
package for complete support.
// Simplified ResizeObserver polyfill (conceptual)// For production, use: https://github.com/que-etc/resize-observer-polyfill(function() { if ('ResizeObserver' in window) return; function ResizeObserver(callback) { this.callback = callback; this.elements = new Map(); var self = this; this._checkSizes = function() { var entries = []; self.elements.forEach(function(lastSize, element) { var rect = element.getBoundingClientRect(); var currentSize = { width: rect.width, height: rect.height }; if (lastSize.width !== currentSize.width || lastSize.height !== currentSize.height) { entries.push({ target: element, contentRect: rect }); self.elements.set(element, currentSize); } }); if (entries.length) { self.callback(entries, self); } }; // Poll for size changes (not performant - use real polyfill) this.interval = setInterval(this._checkSizes, 100); } ResizeObserver.prototype.observe = function(element) { var rect = element.getBoundingClientRect(); this.elements.set(element, { width: rect.width, height: rect.height }); }; ResizeObserver.prototype.unobserve = function(element) { this.elements.delete(element); }; ResizeObserver.prototype.disconnect = function() { clearInterval(this.interval); this.elements.clear(); }; window.ResizeObserver = ResizeObserver;})();// Usagevar resizeObserver = new ResizeObserver(function(entries) { entries.forEach(function(entry) { console.log('Element resized:', entry.target); console.log('New size:', entry.contentRect.width, entry.contentRect.height); });});resizeObserver.observe(document.querySelector('.responsive-element'));
Warning: The IntersectionObserver and ResizeObserver polyfills shown are simplified for demonstration. They use polling which is not performant. For
production, use official polyfills: intersection-observer and resize-observer-polyfill
npm packages.
Key Takeaways - Web APIs
Fetch API: Use whatwg-fetch for full spec compliance with XHR fallback
URLSearchParams: Polyfillable with manual query string parsing
FormData: IE10+ support, iteration methods need polyfills
Blob/FileReader: Convert callback-based APIs to Promises
requestAnimationFrame: Fallback to setTimeout with 16ms delay
Observers: Use official polyfills - custom implementations are complex