What is a higher-order function? | Interview Question

Q: What is a higher-order function?

**Target Audience:** Engineers preparing for JavaScript fundamentals interview questions. **Difficulty:** Easy-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. ## 1. The Core Idea 📌 **Interview term:** a **higher-order function** is a function that does at least one of the following: takes another function as an argument, or returns a function as its result. The name comes from treating functions as ordinary values that can themselves be passed around and returned, exactly like a number or a string -- a direct consequence of JavaScript functions being first-class values. This is a purely structural definition. It says nothing about what the function computes internally -- only about the SHAPE of its inputs and outputs. ## 2. Verified: Taking a Function (the More Comm

The problem

Question presented to candidate: "What is a higher-order function, and can you show me one that both takes and returns a function?"

What a strong answer should cover:

A higher-order function is any function that does at least one of two things: accepts a function as an argument, or returns a function as its result. Either one qualifies -- it does not need to do both.
Built-in array methods like map, filter, reduce, sort, and forEach are all higher-order functions -- they take a callback and use it to process the array.
A function that returns a function -- a factory, a decorator, or a composition helper like compose/pipe -- is a higher-order function in the other direction.
Higher-order functions are what enables function composition: combining small, single-purpose functions into a larger pipeline without writing custom glue code for every combination.
Being a higher-order function is about a function's SHAPE (what it accepts or returns), not about what the function does internally -- it is a structural classification, not a specific algorithm.

Code / implementation expected: Yes -- a runnable snippet showing HOFs that take a function (map/filter/reduce) and one that returns a function (a compose helper), with real chained output.

higher-order

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Solution

Target Audience: Engineers preparing for JavaScript fundamentals interview questions. Difficulty: Easy-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.

1. The Core Idea

📌 Interview term: a higher-order function is a function that does at least one of the following: takes another function as an argument, or returns a function as its result. The name comes from treating functions as ordinary values that can themselves be passed around and returned, exactly like a number or a string -- a direct consequence of JavaScript functions being first-class values.

This is a purely structural definition. It says nothing about what the function computes internally -- only about

The solution is hidden so you can try first

6 min read · 7 sections

Run the code

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

JavaScriptHOFs that take a function: map/filter/reduce (run directly)

Practice

JavaScriptA HOF that returns a function: a compose helper (run directly)

Practice

Discussion

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