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Implement TimeLimitedCache (LeetCode 2622) with expiry

Q: Implement TimeLimitedCache (LeetCode 2622) with expiry

**Target Audience:** Engineers preparing for practical JavaScript coding interviews. **Difficulty:** Medium > **How to read this doc:** the version here stores an `expiresAt` timestamp and checks it lazily, with an injectable clock so every edge case is tested deterministically. The obvious alternative, one `setTimeout` per entry, was also tested and shown to have two real defects: it keeps the Node process alive, and if the old timer is not cleared on overwrite it deletes the NEWER value early. ## 1. The problem, restated A key-value store where every entry carries its own lifetime. Reads must never return an expired value, `set` must report whether it replaced a live entry, and the store should not grow without bound. ## 2. Clarifying questions a strong candidate asks | Question | Why it matters | | :--- | :--- | | Miss and expired result? | LeetCode 2622 specifies `-1`; state the convention so a stored `-1` is not confused with a miss. | | Exact expiry boundary? | Decide whethe

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The problem

Problem, as an interviewer would state it: "Design a cache where set(key, value, duration) stores a value that expires after duration milliseconds. set returns true if an un-expired entry for that key already existed, otherwise false."

Examples:

Code

cache.set(1, 42, 100); // false (new key)
cache.set(1, 50, 50);  // true  (an unexpired entry existed; value and TTL overwritten)
cache.get(1);          // 50

Clarifying questions expected:

What should get return for a missing or expired key?
Does overwriting reset the expiry, and is it the NEW duration that applies?
Should there be a way to count the currently valid keys?

Code / implementation expected: Yes — real, direct proof of the expiry boundary, the overwrite return value, and cleanup, ideally testable without real waiting.

time-limited-cache
expiry
map

Try it yourself

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Solution

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

How to read this doc: the version here stores an expiresAt timestamp and checks it lazily, with an injectable clock so every edge case is tested deterministically. The obvious alternative, one setTimeout per entry, was also tested and shown to have two real defects: it keeps the Node process alive, and if the old timer is not cleared on overwrite it deletes the NEWER value early.

1. The problem, restated

A key-value store where every entry carries its own lifetime. Reads must never return an expired value, set must report whether it replaced a live entry, and the store should not grow without bound.

2. Clarifying questions a strong candidate asks

Question	Why it matters
Miss and expired result?	LeetCode 2622 specifies `-1`; state the

The solution is hidden so you can try first

4 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: a TimeLimitedCache with an injectable clock behaves correctly at the expiry boundary, on overwrite, and when purging expired entries

Practice

Discussion

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