How to Write JavaScript Polyfills from Scratch
How to manually reconstruct native array and function methods to pass JS interviews.
Understand the Prototype Chain
To make a custom method (like myMap) available on all arrays, you must attach it directly to the global prototype: Array.prototype.myMap = function() { ... }.
Master the 'this' Keyword
Inside your polyfill, the 'this' keyword refers to the specific instance that called the method. If the user calls [1,2].myMap(), 'this' points to the array [1,2]. You will iterate over 'this'.
Polyfill Array.map()
Create an empty result array. Loop through 'this' using a standard for-loop. Execute the provided callback function on every iteration passing (this[i], i, this), push the returned value to the result array, and return it.
Polyfill Array.filter()
Create an empty result array. Loop through 'this'. Execute the callback on this[i]. If the callback evaluates to true, push this[i] into the result array. Return the result.
Polyfill Function.bind()
Function.prototype.myBind = function(context, ...args1). Save a reference to 'this' (the original function). Return a new function that gathers ...args2, and inside it, execute originalFn.apply(context, [...args1, ...args2]).
Implement Type Checking
Demonstrate senior-level defensive programming. At the top of your polyfill, check if the caller is the correct type. For bind, write: if (typeof this !== 'function') throw new TypeError('Not a function').
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