Proper TS answers were severely lacking so I figured I'd share my own (even though it could no doubt be even more Typescripty). If you have any suggestions for further TS optimizations, please share them in the comments.
<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 () { const [callbackFn, initialValue] = arguments let accumulator = initialValue // agruments.length === 1 must be used in place of !accumulator because null and undefined are valid initialValues if(!callbackFn || (!this.length &amp;&amp; arguments.length === 1)) { throw new Error() } this.forEach((value) =&gt; { // if not passed initial value, the first element of the array is used as the initial value if (this.indexOf(value) === 0 &amp;&amp; arguments.length === 1) { accumulator = value } else { accumulator = callbackFn(accumulator, value, this.indexOf(value), this) } }) return accumulator}</code></pre>

Typescript Solution w/ Comments (easy to read)

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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)…