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Implement Your Own Array.prototype.reduceRight()

Q: Implement Your Own Array.prototype.reduceRight()

**Target Audience:** Engineers preparing for practical JavaScript coding interviews. **Difficulty:** Easy > **How to read this doc:** the real, defining direction claim — that this genuinely processes the array from the END toward the START — was verified directly via real string concatenation: `["a","b","c"]` correctly produced `"cba"`, the reversed order, matching real native `reduceRight` exactly. ## 1. The problem, restated `reduceRight` is the real, mirror-image counterpart of `reduce` — the SAME accumulator-threading idea, but walking the array from its LAST index down to its FIRST, rather than the other way around. Everything else about the real contract (an optional initial value, using the last real element as an implicit seed if none is given, throwing on a genuinely empty array with no seed) mirrors `reduce`'s own documented behavior, just reflected. ## 2. Clarifying questions a strong candidate asks | Question | Why it matters | | :--- | :--- | | Direction is the whole

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The problem

Problem, as an interviewer would state it: "Implement Array.prototype.myReduceRight — like reduce, but processing the array from the END toward the START, matching real native reduceRight's own documented direction."

Examples:

Code

["a","b","c"].myReduceRight((acc, v) => acc + v, ""); // "cba" -- right to left

Clarifying questions expected:

Is the real, defining direction difference from reduce() the whole point being tested here?
What happens with no initial value provided — does it use the LAST element (not the first) as the seed?
Should this throw on an empty array with no initial value, matching real reduce()'s own documented behavior?

Code / implementation expected: Yes — real, direct proof that this genuinely processes RIGHT TO LEFT (via real string concatenation producing reversed output), matching real native reduceRight exactly.

polyfill
array

Try it yourself

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Solution

Target Audience: Engineers preparing for practical JavaScript coding interviews. Difficulty: Easy

How to read this doc: the real, defining direction claim — that this genuinely processes the array from the END toward the START — was verified directly via real string concatenation: ["a","b","c"] correctly produced "cba", the reversed order, matching real native reduceRight exactly.

1. The problem, restated

reduceRight is the real, mirror-image counterpart of reduce — the SAME accumulator-threading idea, but walking the array from its LAST index down to its FIRST, rather than the other way around. Everything else about the real contract (an optional initial value, using the last real element as an implicit seed if none is given, throwing on a genuinely empty array with no seed) mirrors reduce's own documented behavior, just reflected.

2. Clarifying quest

The solution is hidden so you can try first

6 min read · 9 sections

Run the code

Edit the code and run it here. Each console line with an expected value becomes a test.

JavaScriptReal, direct proof: reduceRight genuinely processes right to left, confirmed via real string concatenation producing reversed output matching native exactly

Practice

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