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| 1 | +```js |
| 2 | +Array.prototype.myReduce = function (callbackFn, initialValue) { |
| 3 | + if (typeof callbackFn !== 'function') { |
| 4 | + throw new TypeError(callbackFn + ' is not a function'); |
| 5 | + } |
| 6 | + |
| 7 | + const arr = this; // `this` refers to the array on which myReduce is called |
| 8 | + let accumulator = initialValue; // Initialize the accumulator |
| 9 | + let startIndex = 0; // Starting index for iteration |
| 10 | + |
| 11 | + // If initialValue is not provided, use the first element as the initial value |
| 12 | + if (accumulator === undefined) { |
| 13 | + if (arr.length === 0) { |
| 14 | + throw new TypeError('Reduce of empty array with no initial value'); |
| 15 | + } |
| 16 | + accumulator = arr[0]; |
| 17 | + startIndex = 1; // Skip the first element since it's now the accumulator |
| 18 | + } |
| 19 | + |
| 20 | + // Iterate through the array elements and apply the callback function |
| 21 | + for (let i = startIndex; i < arr.length; i++) { |
| 22 | + accumulator = callbackFn(accumulator, arr[i], i, arr); |
| 23 | + } |
| 24 | + |
| 25 | + return accumulator; // Return the accumulated value |
| 26 | +}; |
| 27 | +``` |
| 28 | +### Explanation |
| 29 | +Check if callbackFn is a function: We first check if the provided callbackFn is a valid function. If not, we throw a TypeError. |
| 30 | + |
| 31 | +#### Handle initialValue: |
| 32 | + |
| 33 | +* If initialValue is provided, we use it to initialize the accumulator. |
| 34 | +* If no initialValue is provided, we take the first element of the array (arr[0]) as the accumulator. |
| 35 | +* However, if the array is empty and no initialValue is provided, we throw a TypeError. |
| 36 | + |
| 37 | +#### Iterate over the array: We use a for loop to iterate over the elements of the array starting at startIndex. For each element, we call the callbackFn with the following arguments: |
| 38 | + |
| 39 | +* accumulator (the accumulated value so far), |
| 40 | +* the current element (arr[i]), |
| 41 | +* the current index (i), |
| 42 | +* the entire array (arr). |
| 43 | + |
| 44 | +#### Update the accumulator: The result of the callbackFn is assigned back to the accumulator. |
| 45 | + |
| 46 | +#### Return the result: Finally, we return the accumulated value (accumulator). |
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