How do you implement a throttle function? | Interview Question

Q: How do you implement a throttle function?

**Target Audience:** Engineers preparing for JavaScript / frontend-implementation interview questions. **Difficulty:** Medium > **How to read this doc:** Concepts are explained in plain language first, then tagged with 📌 Interview term: . Every result below was actually run on Node.js v24.19.0 — the printed output is real, captured output, not illustrative sample text. For the conceptual distinction between debounce and throttle before diving into code, see What is debouncing and throttling? For a version of this exact function extended with configurable leading/trailing edges, see How would you implement a throttle function with configurable leading- and trailing-edge execution? ## 1. Why This Even Matters — A Story First Picture an airport security lane with a strict rule: the

01

Understand the problem

Question presented to candidate: "Implement a throttle function from scratch. Walk me through why it caps the call rate, then show me it actually limiting a burst of rapid calls."

What a strong answer should cover:

A closure holding a boolean cooldown flag (or, in an alternative implementation, a "last run timestamp").
On each call: if currently in cooldown, ignore the call entirely; otherwise run the wrapped function immediately and enter cooldown for the configured interval.
This means the FIRST call in a burst fires immediately (the "leading edge") by default, unlike debounce which always waits.
The cooldown-flag version and the last-run-timestamp version are both correct, common implementations with a real tradeoff between them, not just stylistic variants.
Real verification: proving with actual timestamps that a rapid burst produces a genuinely capped number of effect calls, spread at roughly the configured interval.

Clarifying questions expected:

"Should this fire on the leading edge, the trailing edge, or both?" (a strong candidate flags that the simplest version only does leading, and that configurable edges are a natural, common follow-up)

Code / implementation expected: Yes — a full working throttle implementation, executed with a rapid-fire burst and real observed timestamps proving the capped call rate.

throttle

02

Attempt it yourself

Sketch your approach before reading the solution — that's what interviews test.

Nudge consolestandby

Stuck? Beam a request up — the console returns a conceptual nudge that guides your logic without spoiling the implementation.

03

Study the solution

Target Audience: Engineers preparing for JavaScript / frontend-implementation interview questions. Difficulty: Medium

How to read this doc: Concepts are explained in plain language first, then tagged with 📌 Interview term:. Every result below was actually run on Node.js v24.19.0 — the print

Solution ready — 2 min read

Classified // press E to declassify

04

Run the code

JSThrottle from scratch: 15 rapid calls capped to 4 (run directly)

Practice

05

Join the discussion

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