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Polyfill Array.prototype.findLast() and findLastIndex()

Q: Polyfill Array.prototype.findLast() and findLastIndex()

**Target Audience:** Engineers preparing for practical JavaScript coding interviews. **Difficulty:** Easy > **How to read this doc:** every claim was verified DIRECTLY against the REAL, native `findLast`/`findLastIndex`, confirming identical output on a real matching case and identical "no match" conventions (`undefined` and `-1` respectively). ## 1. The problem, restated `findLast`/`findLastIndex` search an array from its LAST index toward its FIRST, returning the first (from that reversed direction) element (or index) satisfying the callback — effectively "the last element in the array matching the condition," genuinely different from `find`/`findIndex`'s own forward-searching, first-match behavior. ## 2. Clarifying questions a strong candidate asks | Question | Why it matters | | :--- | :--- | | No-match return values? | `findLast` returns `undefined`, `findLastIndex` returns `-1` — mirroring `find`/`findIndex`'s own real, documented conventions exactly. | | Genuinely need to s

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The problem

Problem, as an interviewer would state it: "Implement Array.prototype.myFindLast and myFindLastIndex — like find/findIndex, but searching from the END of the array toward the START, returning the LAST matching element."

Examples:

Code

[5, 12, 8, 130, 44].myFindLast(n => n > 10); // 44 -- the LAST element greater than 10

Clarifying questions expected:

What is the real, correct "no match found" return value for each — matching find()'s own undefined, and findIndex()'s own -1, respectively?
Does this genuinely need to search from the end for efficiency, or would reversing the array first and using find() be equally correct?
Should real sparse-array holes be handled the same way as find()'s own real, documented behavior (which does NOT skip holes, unlike map/filter)?

Code / implementation expected: Yes — real, direct proof against the ACTUAL native findLast/findLastIndex, confirming identical output including the correct "no match" conventions.

array

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Solution

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

How to read this doc: every claim was verified DIRECTLY against the REAL, native findLast/findLastIndex, confirming identical output on a real matching case and identical "no match" conventions (undefined and -1 respectively).

1. The problem, restated

findLast/findLastIndex search an array from its LAST index toward its FIRST, returning the first (from that reversed direction) element (or index) satisfying the callback — effectively "the last element in the array matching the condition," genuinely different from find/findIndex's own forward-searching, first-match behavior.

2. Clarifying questions a strong candidate asks

Question	Why it matters
No-match return values?	`findLast` returns `undefined`, `findLastIndex` returns `-1

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: findLast()/findLastIndex() match the real native methods exactly, including the correct no-match conventions

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