Learn Polyfill of JS Reduce with ChatGPT

// callback -> previousValue, currentValue, index, array
function myReduce(callback, initialValue) {
    const array = this
    if(!array) throw new Error("Array is not defined")
    const arrayLength = this.length
    let index = 0

    if(!arrayLength) {
        if(!initialValue) throw new Error("You need to 
pass initial value if array is empty")
        return initialValue
    let accumulator = initialValue ?? array[index++]
    while(index < arrayLength) {
        accumulator = callback(accumulator, array[index], index, array)

    return accumulator

Array.prototype.myReduce = myReduce

const arr = undefined
const intValue = 1

const callback = (prev, currnt) => prev + currnt

console.log(arr.myReduce(callback, intValue))

Me: is this polyfill correct for js reduce?

ChatGPT: The provided polyfill for Array.prototype.myReduce looks generally correct. However, there are a few improvements and corrections that could be made:

  1. Checking for this existence: Instead of checking if array is not null or undefined, it's better to use if (!Array.isArray(this)) to check if this is indeed an array.

  2. Error messages: Error messages should ideally provide more context. For instance, if initialValue is not provided when the array is empty, you should specify that in the error message.

  3. Checking for callback existence: It's advisable to check if the callback function is provided.

Here's an updated version with these improvements:

function myReduce(callback, initialValue) {
    if (!Array.isArray(this)) {
        throw new Error("myReduce can only be used on arrays.");

    const array = this;
    const arrayLength = array.length;
    let index = 0;

    if (arrayLength === 0) {
        if (initialValue === undefined) {
            throw new Error("Reduce of empty array with no initial value.");
        return initialValue;

    if (typeof callback !== "function") {
        throw new TypeError("Callback must be a function.");

    let accumulator = initialValue !== undefined ? initialValue : array[index++];

    while (index < arrayLength) {
        accumulator = callback(accumulator, array[index], index, array);

    return accumulator;

Array.prototype.myReduce = myReduce;

// Example usage
const arr = [1, 2, 3, 4];
const initialValue = 0;

const callback = (prev, current) => prev + current;

console.log(arr.myReduce(callback, initialValue)); // Output: 10

This updated version ensures better error handling and checks for callback existence, providing more robustness.

Reference: https://jscafe.dev/index.php/q14-polyfill-of-array-reduce/

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