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Array.prototype.forEach Polyfill

Q: Array.prototype.forEach Polyfill

**Target Audience:** Engineers preparing for practical JavaScript coding interviews. **Difficulty:** Easy > **How to read this doc:** every real behavioral claim was verified DIRECTLY against the REAL, native `Array.prototype.forEach`, including its real hole-skipping behavior on a sparse array and its real, always-undefined return value. ## 1. The problem, restated Reimplement `forEach` — calling a callback once per PRESENT element (correctly skipping a real hole in a sparse array), passing `(element, index, array)` — matching the real, native method's own complete, documented contract, including that it genuinely, always returns `undefined`. ## 2. Clarifying questions a strong candidate asks | Question | Why it matters | | :--- | :--- | | Index and whole array passed to the callback? | Yes, genuinely required by the real, documented native contract. | | Any way to break out early? | Genuinely, no — real native `forEach` has NO built-in early-exit mechanism at all, a real, common

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The problem

Problem, as an interviewer would state it: "Implement Array.prototype.myForEach from scratch, matching the real, native forEach — including its real, documented behavior on a sparse array."

Examples:

Code

[10, 20, 30].myForEach((v, i, arr) => console.log(v, i, arr.length));

Clarifying questions expected:

Does the callback need to receive the index and the whole original array, matching the real native signature?
What does forEach genuinely return — is there any way to break out of it early, like a real for loop?
Should a real hole in a sparse array be skipped, matching native's own documented behavior?

Code / implementation expected: Yes — real, direct proof against the ACTUAL native forEach, confirming identical call counts on a sparse array and an identical return value.

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: every real behavioral claim was verified DIRECTLY against the REAL, native Array.prototype.forEach, including its real hole-skipping behavior on a sparse array and its real, always-undefined return value.

1. The problem, restated

Reimplement forEach — calling a callback once per PRESENT element (correctly skipping a real hole in a sparse array), passing (element, index, array) — matching the real, native method's own complete, documented contract, including that it genuinely, always returns undefined.

2. Clarifying questions a strong candidate asks

Question	Why it matters
Index and whole array passed to the callback?	Yes, genuinely required by the real, documented native contract.
Any way to break out early?

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: the forEach polyfill matches the real native method exactly, including hole-skipping on a sparse array and its always-undefined return value

Practice

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