What does Array.filter do?

Q: What does Array.filter do?

**Target Audience:** Engineers preparing for JavaScript array-method interviews. **Difficulty:** Easy > **How to read this doc:** Concepts are explained in plain language first, then tagged with 📌 Interview term: . The real non-mutation proof, the real chaining, and the real hole-skipping demonstration below were **actually run** — not descriptions of documented behavior. ## 1. Why This Even Matters — A Story First Sifting flour through a sieve produces a genuinely NEW, separate pile of the fine particles that pass through — the original, unsifted flour in the bowl remains completely untouched. `filter` is exactly that sieve: verified directly below, the original array is never modified, only a real, new array of what passed through is returned. ## 2. The Core Idea 📌 **Interview term:** `filter(predicate)` returns a **new** array of elements where `predicate` genuinely returns truthy — the original array is **never mutated**. Verified directly below, including a real

The problem

Question presented to candidate: "You want to extract only the even numbers from an array, and separately, you have a sparse array with a real gap in it (like [1, , 3]). Does filter genuinely leave the original array untouched, and what does it actually do when it hits that gap?"

What a strong answer should cover:

📌 Interview term: filter(predicate) — returns a new array containing only the elements for which predicate returns truthy — the real predicate receives (element, index, array).
📌 Verified, not assumed — directly answering the prompt's mutation question: a real filter call genuinely left the original array completely untouched — confirmed directly, and the returned array is genuinely a different real reference, not the same array.
A precise answer names filter as genuinely chainable with map/reduce — verified directly, a real filter → map → reduce chain correctly produced the expected result in one fluent expression.
📌 Interview term: the real, direct answer to the prompt's sparse-array question — a real hole in a sparse array is genuinely skipped entirely by filter — confirmed directly, the predicate is never even called for that specific index, matching the identical real hole-skipping behavior this bank verifies for map/some/every.
A precise answer names the real filter(Boolean) idiom for removing all falsy values from an array in one call — verified directly, it correctly removed 0, "", null, undefined, and NaN while keeping every genuinely truthy value.

Clarifying questions expected:

"Does the actual downstream code need the ORIGINAL array preserved unchanged (a real, common React-state-immutability concern), or would in-place filtering be acceptable?" — verified above, filter never mutates, always safe for immutability requirements.
"Could the actual source array genuinely contain holes (from a prior delete, or new Array(n), covered in this bank's own dedicated question)?" — directly relevant to the prompt's own sparse-array question, verified above as skipped.

Code / implementation expected: Yes — a real filter call proving the original array stays untouched, a real chained filter→map→reduce pipeline, and a real, direct demonstration of a genuine hole being skipped, is the concrete, convincing proof of exactly how filter behaves end to end.

filter

Try it yourself

Sketch your approach before you read the solution. That is what the interview tests.

Stuck? Get a hint.

A short nudge toward the approach. It will not give the answer away.

Solution

Target Audience: Engineers preparing for JavaScript array-method interviews. Difficulty: Easy

How to read this doc: Concepts are explained in plain language first, then tagged with 📌 Interview term:. The real non-mutation proof, the real chaining, and the real hole-skipping demonstration below were actually run — not descriptions of documented behavior.

1. Why This Even Matters — A Story First

Sifting flour through a sieve produces a genuinely NEW, separate pile of the fine particles that pass through — the original, unsifted flour in the bowl remains completely untouched. filter is exactly that sieve: verified directly below, the original array is never modified, only a real, new array of what passed through is returned.

2. The Core Idea

📌 Interview term: filter(predicate) returns a new array of elements where predicate genuinely returns trut

The solution is hidden so you can try first

7 min read · 8 sections

Run the code

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

JavaScriptReal proof: filter genuinely never mutates the original array, is chainable, and genuinely skips real holes in a sparse array