How do you implement a throttle function?

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

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

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 / 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 officer waves exactly one traveler through, then looks away for three full seconds no matter how many peopl

The solution is hidden so you can try first

7 min read · 7 sections

Run the code

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