<pre class="shiki github-dark shaku"><code>// copied from lib.es5.d.tsdeclare interface Array&lt;T&gt; { myReduce(callbackfn: (previousValue: T, currentValue: T, currentIndex: number, array: T[]) =&gt; T): T; myReduce(callbackfn: (previousValue: T, currentValue: T, currentIndex: number, array: T[]) =&gt; T, initialValue: T): T; myReduce&lt;U&gt;(callbackfn: (previousValue: U, currentValue: T, currentIndex: number, array: T[]) =&gt; U, initialValue: U): U}Array.prototype.myReduce = function&lt;T, U&gt; (callbackfn: (previousValue: U | T, currentValue: T, currentIndex: number, array: T[]) =&gt; U | T, initialValue?: U | T) { const thisArr = this if(!Array.isArray(thisArr) || !callbackfn){ throw Error('Should call by array and should pass callback fn.') } const thisArrLength = thisArr.length // record static thisArr length const hasInitialValue = arguments.length &gt; 1 if(thisArrLength === 0 &amp;&amp; !hasInitialValue) { throw Error('Should pass initial value if arr is empty.') } // if has initial value, should use it // or should use first element of thisArr and also need to move forward inital index to make the for loop right let accumulatedValue = hasInitialValue ? initialValue : thisArr[0] let initialIndex = hasInitialValue ? 0 : 1 for(let i = initialIndex; i &lt; thisArrLength; i++) { if(i in thisArr) { accumulatedValue = callbackfn(accumulatedValue, thisArr[i], i, thisArr) // callbackfn will return accumulated value for this iteration } } return accumulatedValue}console.log([1,2,3].myReduce((sum, item) =&gt; sum + item)) // 6console.log([1,2,3].myReduce((sum, item) =&gt; sum + item, 0)) // 6console.log([1,2,3].myReduce((sum, item) =&gt; sum + item, 4)) // 10</code></pre>

Implement full reduce with explanned comments

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Array.prototype.reduce() is a handy method to process arrays. Here is a simple task - Could you implement it by yourself? [1,2,3].myReduce((sum, item)…