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Implement Array.prototype.some() and every() From Scratch

Q: Implement Array.prototype.some() and every() From Scratch

**Target Audience:** Engineers preparing for practical JavaScript coding interviews. **Difficulty:** Easy > **How to read this doc:** the real, DEFINING behavior this question tests — that both methods genuinely SHORT-CIRCUIT rather than checking every element — was verified directly with a real, logged list confirming exactly which elements were actually checked before each method correctly stopped early. ## 1. The problem, restated `some` returns `true` the INSTANT any element satisfies the callback (never checking further elements once found); `every` returns `false` the INSTANT any element FAILS the callback (never checking further elements once a failure is found) — both genuinely stop early rather than exhaustively checking the whole array, a real, meaningful efficiency property, not just a correctness detail. ## 2. Clarifying questions a strong candidate asks | Question | Why it matters | | :--- | :--- | | Genuine short-circuiting required? | Yes — this is the real, definin

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The problem

Problem, as an interviewer would state it: "Implement Array.prototype.mySome and myEvery from scratch — matching real native behavior, including that both genuinely SHORT-CIRCUIT rather than checking every element."

Examples:

Code

[1,2,3,4,5].mySome(n => n === 3); // true, and STOPS checking after finding it

Clarifying questions expected:

Do both methods need to genuinely stop early once the answer is already determined, or is checking every element acceptable?
What is the real, correct result for an empty array — for some(), and separately for every()?
Do these two methods need to be implemented as genuinely separate functions, or could one be derived from the other?

Code / implementation expected: Yes — real, direct proof of correct output matching native, PLUS real, direct proof of genuine short-circuiting via a logged list of exactly which elements were checked before stopping.

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 behavior this question tests — that both methods genuinely SHORT-CIRCUIT rather than checking every element — was verified directly with a real, logged list confirming exactly which elements were actually checked before each method correctly stopped early.

1. The problem, restated

some returns true the INSTANT any element satisfies the callback (never checking further elements once found); every returns false the INSTANT any element FAILS the callback (never checking further elements once a failure is found) — both genuinely stop early rather than exhaustively checking the whole array, a real, meaningful efficiency property, not just a correctness detail.

2. Clarifying questions a strong candidate asks

| Question | Why

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: some()/every() genuinely short-circuit, confirmed via a real logged list of exactly which elements were checked before stopping

Practice

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