How to Write Custom Polyfills

1. Polyfill Structure and Template Patterns

Component	Pattern	Purpose	Example
Feature Detection	`if (!Type.prototype.method)`	Check if polyfill needed	Prevent overwriting native implementation
IIFE Wrapper	`(function() { ... })()`	Isolate scope and prevent pollution	Avoid variable leakage to global scope
Strict Mode	`'use strict';`	Enforce better coding practices	Catch common errors early
Type Checking	`if (this == null) throw TypeError`	Validate context and arguments	Match native behavior for errors
ToObject Coercion	`var O = Object(this)`	Convert this to object safely	Handle primitives correctly
Length Coercion	`len = O.length >>> 0`	Convert length to uint32	Match array length behavior

Example: Complete polyfill template structure

// Standard Polyfill Template
(function() {
    'use strict';
    
    // Feature detection - exit if already exists
    if (Array.prototype.includes) {
        return;
    }
    
    // Polyfill implementation
    Array.prototype.includes = function(searchElement, fromIndex) {
        // Step 1: Type check - throw if this is null/undefined
        if (this == null) {
            throw new TypeError('Array.prototype.includes called on null or undefined');
        }
        
        // Step 2: Convert this to object
        var O = Object(this);
        
        // Step 3: Get and convert length to unsigned 32-bit integer
        var len = parseInt(O.length) || 0;
        
        // Step 4: Handle edge cases
        if (len === 0) {
            return false;
        }
        
        // Step 5: Process fromIndex parameter
        var n = parseInt(fromIndex) || 0;
        var k = Math.max(n >= 0 ? n : len - Math.abs(n), 0);
        
        // Step 6: Main algorithm
        while (k < len) {
            var currentElement = O[k];
            // Handle NaN comparison (NaN === NaN should be true)
            if (searchElement === currentElement ||
                (searchElement !== searchElement && currentElement !== currentElement)) {
                return true;
            }
            k++;
        }
        
        // Step 7: Return result
        return false;
    };
})();

Note: Always wrap polyfills in an IIFE to avoid polluting global scope. Use strict mode to catch errors and ensure feature detection runs first to avoid overwriting native implementations.

2. Prototype Extension (Array.prototype, String.prototype)

Practice	Do This	Don't Do This	Reason
Check Existence	`if (!proto.method)`	Unconditional assignment	Preserve native implementation
Use defineProperty	`Object.defineProperty()`	`proto.method = fn`	Control enumerability and writability
Non-enumerable	`enumerable: false`	`enumerable: true`	Don't break for...in loops
Configurable	`configurable: true`	`configurable: false`	Allow future modifications
Test Context	`if (this == null) throw`	Assume this is valid	Match native error behavior
Handle Edge Cases	Check length, NaN, undefined	Assume happy path	Spec compliance and robustness

Example: Proper prototype extension with defineProperty

// Good: Using Object.defineProperty
if (!Array.prototype.find) {
    Object.defineProperty(Array.prototype, 'find', {
        value: function(predicate) {
            if (this == null) {
                throw new TypeError('this is null or not defined');
            }
            if (typeof predicate !== 'function') {
                throw new TypeError('predicate must be a function');
            }
            
            var O = Object(this);
            var len = O.length >>> 0;
            var thisArg = arguments[1];
            
            for (var k = 0; k < len; k++) {
                if (k in O) {
                    var kValue = O[k];
                    if (predicate.call(thisArg, kValue, k, O)) {
                        return kValue;
                    }
                }
            }
            return undefined;
        },
        configurable: true,
        writable: true,
        enumerable: false  // Critical: prevents for-in enumeration
    });
}

// Bad: Direct assignment (enumerable by default)
// Array.prototype.find = function() { ... };  // DON'T DO THIS

Warning: Never extend prototypes of native objects without feature detection. Always use Object.defineProperty with enumerable: false to avoid breaking existing code.

3. Static Method Polyfills (Object.assign, Array.from)

Type	Pattern	Check Method	Example
Object Static	`Object.assign = function() {}`	`if (!Object.assign)`	Object.keys, Object.values, Object.entries
Array Static	`Array.from = function() {}`	`if (!Array.from)`	Array.from, Array.of, Array.isArray
Number Static	`Number.isNaN = function() {}`	`if (!Number.isNaN)`	Number.isFinite, Number.parseInt
String Static	`String.fromCodePoint = function() {}`	`if (!String.fromCodePoint)`	String.raw, String.fromCodePoint
Math Static	`Math.trunc = function() {}`	`if (!Math.trunc)`	Math.sign, Math.trunc, Math.cbrt

Example: Static method polyfills

// Object.assign polyfill
if (!Object.assign) {
    Object.assign = function(target) {
        'use strict';
        if (target == null) {
            throw new TypeError('Cannot convert undefined or null to object');
        }
        
        var to = Object(target);
        
        for (var index = 1; index < arguments.length; index++) {
            var nextSource = arguments[index];
            
            if (nextSource != null) {
                for (var nextKey in nextSource) {
                    if (Object.prototype.hasOwnProperty.call(nextSource, nextKey)) {
                        to[nextKey] = nextSource[nextKey];
                    }
                }
            }
        }
        return to;
    };
}

// Array.from polyfill
if (!Array.from) {
    Array.from = function(arrayLike, mapFn, thisArg) {
        var C = this;
        var items = Object(arrayLike);
        var len = items.length >>> 0;
        
        var A = typeof C === 'function' ? 
                Object(new C(len)) : new Array(len);
        
        var k = 0;
        while (k < len) {
            var kValue = items[k];
            if (mapFn) {
                A[k] = typeof thisArg === 'undefined' ? 
                       mapFn(kValue, k) : 
                       mapFn.call(thisArg, kValue, k);
            } else {
                A[k] = kValue;
            }
            k++;
        }
        A.length = len;
        return A;
    };
}

// Number.isNaN polyfill (more accurate than global isNaN)
if (!Number.isNaN) {
    Number.isNaN = function(value) {
        // NaN is the only value that is not equal to itself
        return value !== value;
    };
}

4. Object.defineProperty for Non-enumerable Methods

Descriptor	Value	Effect	Use Case
value	Any value	The actual function or value	Set the method implementation
writable	`true/false`	Can property be changed with assignment	Usually true for polyfills
enumerable	`false`	Appears in for...in loops	Must be false to match native behavior
configurable	`true`	Can descriptor be changed/deleted	Allow future updates
get	Function	Getter function for property	Computed properties
set	Function	Setter function for property	Validation on assignment

Example: Advanced defineProperty usage patterns

// Data descriptor (value + writable)
Object.defineProperty(Array.prototype, 'last', {
    get: function() {
        return this[this.length - 1];
    },
    set: function(value) {
        this[this.length - 1] = value;
    },
    enumerable: false,
    configurable: true
});

// Usage: arr.last returns last element
var arr = [1, 2, 3];
console.log(arr.last);  // 3
arr.last = 5;
console.log(arr.last);  // 5

// Batch property definition
if (!Object.defineProperties) {
    Object.defineProperties = function(obj, properties) {
        for (var prop in properties) {
            if (properties.hasOwnProperty(prop)) {
                Object.defineProperty(obj, prop, properties[prop]);
            }
        }
        return obj;
    };
}

// Define multiple properties at once
Object.defineProperties(String.prototype, {
    "trim": {
        value: function() {
            return this.replace(/^\s+|\s+$/g, "");
        },
        writable: true,
        enumerable: false,
        configurable: true
    },
    "trimStart": {
        value: function() {
            return this.replace(/^\s+/, "");
        },
        writable: true,
        enumerable: false,
        configurable: true
    }
});

Note: Use accessor descriptors (get/set) for computed properties. Use data descriptors (value/writable) for methods. Never mix both in the same descriptor.

5. UMD and ES Module Polyfill Patterns

Format	Detection	Export Pattern	Use Case
CommonJS	`typeof module !== 'undefined'`	`module.exports = polyfill`	Node.js environments
AMD	`typeof define === 'function' && define.amd`	`define([], factory)`	RequireJS, legacy browsers
ES Module	Separate .mjs file	`export default polyfill`	Modern bundlers, native ESM
Global	Fallback for all	`window.polyfill = ...`	Browser script tags
UMD	Combines all patterns	Check order: AMD, CommonJS, Global	Universal compatibility

Example: UMD wrapper pattern

// Universal Module Definition (UMD) Pattern
(function(root, factory) {
    'use strict';
    
    // AMD (RequireJS)
    if (typeof define === 'function' && define.amd) {
        define([], factory);
    }
    // CommonJS (Node.js)
    else if (typeof module === 'object' && module.exports) {
        module.exports = factory();
    }
    // Browser globals
    else {
        root.myPolyfill = factory();
    }
}(typeof self !== 'undefined' ? self : this, function() {
    'use strict';
    
    // Feature detection
    if ('myFeature' in window) {
        return window.myFeature;
    }
    
    // Polyfill implementation
    function MyPolyfill() {
        // Implementation here
    }
    
    MyPolyfill.prototype.method = function() {
        // Method implementation
    };
    
    // Install globally if needed
    if (typeof window !== 'undefined') {
        window.myFeature = MyPolyfill;
    }
    
    // Return for module systems
    return MyPolyfill;
}));

// ES Module format (separate file)
// polyfill.mjs
export default function polyfill() {
    if (!Array.prototype.at) {
        Array.prototype.at = function(index) {
            var len = this.length;
            var k = index >= 0 ? index : len + index;
            return (k >= 0 && k < len) ? this[k] : undefined;
        };
    }
}

// Usage in ES modules
// import polyfill from './polyfill.mjs';
// polyfill();

Note: For maximum compatibility, use UMD pattern in polyfills. Modern projects can use ES modules (.mjs) with build tool transpilation for legacy support.

6. Idempotency and Safe Polyfill Execution

Pattern	Implementation	Purpose	Benefit
Existence Check	`if (!Type.prototype.method)`	Only add if missing	Prevent double-loading issues
Early Return	`if (hasFeature) return;`	Exit immediately if native exists	Zero overhead for modern browsers
Version Check	`if (polyfill.version) return;`	Track polyfill installation	Avoid conflicts with other polyfills
Strict Equality	`feature === undefined`	Precise undefined checking	Distinguish from null or falsy values
IIFE Isolation	`(function(){ ... })()`	Create private scope	Prevent variable name collisions
Namespace Guard	`window.POLYFILLS = {}`	Track loaded polyfills	Coordinate multiple polyfill scripts

Example: Idempotent polyfill patterns

// Pattern 1: Simple existence check
(function() {
    'use strict';
    
    // Early return if already exists
    if (Array.prototype.includes) {
        return;
    }
    
    // Polyfill implementation
    Array.prototype.includes = function(search) {
        // Implementation...
    };
})();

// Pattern 2: Version tracking
(function() {
    'use strict';
    
    // Check if already loaded
    if (window.__POLYFILLS__ && window.__POLYFILLS__.arrayIncludes) {
        return;
    }
    
    // Initialize polyfill registry
    window.__POLYFILLS__ = window.__POLYFILLS__ || {};
    
    if (!Array.prototype.includes) {
        Array.prototype.includes = function(search) {
            // Implementation...
        };
    }
    
    // Mark as loaded
    window.__POLYFILLS__.arrayIncludes = '1.0.0';
})();

// Pattern 3: Safe multiple execution
(function(global) {
    'use strict';
    
    // Create polyfill only once
    function createPolyfill() {
        // Check each feature individually
        if (!Array.prototype.find) {
            Object.defineProperty(Array.prototype, 'find', {
                value: function(predicate) { /* ... */ },
                configurable: true,
                writable: true,
                enumerable: false
            });
        }
        
        if (!Array.prototype.findIndex) {
            Object.defineProperty(Array.prototype, 'findIndex', {
                value: function(predicate) { /* ... */ },
                configurable: true,
                writable: true,
                enumerable: false
            });
        }
    }
    
    // Safe to call multiple times - only executes missing polyfills
    createPolyfill();
    
})(typeof window !== 'undefined' ? window : global);

Key Takeaways - Polyfill Writing

Always wrap polyfills in IIFE with strict mode
Use feature detection before adding any polyfill
Set enumerable: false when extending prototypes
Match native error behavior with proper type checking
Use UMD pattern for maximum compatibility
Make polyfills idempotent - safe to load multiple times