What arguments does the map callback receive in JavaScript?
Three arguments: the current element, the current index, and the array itself. Most callbacks only use the first one, but the polyfill should pass all three to match the spec.
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More FAQs in Writing a Polyfill for map in JavaScript
Add a method to Array.prototype that loops through the array, calls the callback with (element, index, array), pushes the result to a new array, and returns the new array. Use let in the loop and do not mutate the original.
No. map returns a new array with the results. The original array is unchanged. A polyfill should also not mutate the original array.
An optional second argument that sets this inside the callback. Support it with callback.call(thisArg, element, index, array) instead of callback(element, index, array).
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