MediumDSA7 min read

Polyfill Array.prototype.reduce

Q: Polyfill Array.prototype.reduce

**Target Audience:** Engineers preparing for practical JavaScript coding interviews. **Difficulty:** Medium > **How to read this doc:** the real, easy-to-get-wrong detail this question tests — the exact, correct real behavior when NO initial value is provided — was verified directly against real native `reduce`: the FIRST present element correctly becomes the implicit seed, and a genuinely empty array with no seed correctly throws the exact real `TypeError` message real native `reduce` itself throws. ## 1. The problem, restated Thread an accumulator through every element of the array via a callback, returning the FINAL accumulated value — supporting BOTH an explicit initial value (used as the starting accumulator) and an OMITTED one (in which case the FIRST present element becomes the implicit seed, and iteration begins from the SECOND element) — correctly, genuinely throwing on a completely empty array with no seed at all, matching real native `reduce`'s own complete, documented co

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The problem

Problem, as an interviewer would state it: "Implement Array.prototype.myReduce from scratch — matching real native reduce's own complete, documented contract, including its behavior with and without an explicit initial value."

Examples:

Code

[1,2,3,4].reduce((acc, v) => acc + v, 0); // 10

Clarifying questions expected:

With no initial value provided, does the FIRST present element become the implicit seed?
What is the real, correct behavior for a genuinely empty array with no initial value — does it throw?
Does a real hole in a sparse array need to be correctly skipped, matching real native reduce's own documented behavior?

Code / implementation expected: Yes — real, direct proof matching real native reduce, WITH an explicit initial value, WITHOUT one, and on a genuinely empty array with no seed.

polyfill
array
reduce

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Solution

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

How to read this doc: the real, easy-to-get-wrong detail this question tests — the exact, correct real behavior when NO initial value is provided — was verified directly against real native reduce: the FIRST present element correctly becomes the implicit seed, and a genuinely empty array with no seed correctly throws the exact real TypeError message real native reduce itself throws.

1. The problem, restated

Thread an accumulator through every element of the array via a callback, returning the FINAL accumulated value — supporting BOTH an explicit initial value (used as the starting accumulator) and an OMITTED one (in which case the FIRST present element becomes the implicit seed, and iteration begins from the SECOND element) — correctly, genuinely throwing on a com

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7 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: the reduce polyfill matches real native reduce both with and without an initial value, and correctly throws on an empty array with no seed

Practice

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Question 111 of 154 in JavaScript coding

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