MediumFrontend6 min read

Implement compose() and pipe() for function composition

Q: Implement compose() and pipe() for function composition

**Target Audience:** Engineers preparing for practical JavaScript coding interviews. **Difficulty:** Medium > **How to read this doc:** the claim that compose and pipe produce the IDENTICAL final result when given the same functions in REVERSED order was verified directly with real computed output (64, both ways, for the same input). ## 1. The problem, restated `compose(f, g, h)(x)` applies RIGHT TO LEFT: `f(g(h(x)))`. `pipe(f, g, h)(x)` applies LEFT TO RIGHT: `h(g(f(x)))`... more precisely, `pipe` applies its arguments in the order GIVEN, left to right, while `compose` applies its arguments in REVERSE, right to left — the real, standard, math-notation-inspired convention (`compose` mirrors mathematical function composition `(f ∘ g)(x) = f(g(x))`). ## 2. Clarifying questions a strong candidate asks | Question | Why it matters | | :--- | :--- | | compose direction? | Right-to-left — matching real, standard math notation `f ∘ g`, a real, common point of confusion worth confirming ex

1k views

93 of 154

The problem

Problem, as an interviewer would state it: "Implement compose(...fns) and pipe(...fns) — both combine multiple single-argument functions into one, differing only in the ORDER they apply."

Examples:

Code

compose(square, double, addOne)(3); // square(double(addOne(3))) -- right to left
pipe(addOne, double, square)(3);    // square(double(addOne(3))) -- left to right, SAME result

Clarifying questions expected:

Which direction does "compose" apply by real, standard convention — right-to-left, or left-to-right?
Do the combined functions need to support multiple arguments, or just a single value threaded through?
What should an empty fns list (compose()) return?

Code / implementation expected: Yes — real, direct proof that compose and pipe produce the identical result for the same functions with reversed call order.

compose
pipe
functional

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 practical JavaScript coding interviews. Difficulty: Medium

How to read this doc: the claim that compose and pipe produce the IDENTICAL final result when given the same functions in REVERSED order was verified directly with real computed output (64, both ways, for the same input).

1. The problem, restated

compose(f, g, h)(x) applies RIGHT TO LEFT: f(g(h(x))). pipe(f, g, h)(x) applies LEFT TO RIGHT: h(g(f(x)))... more precisely, pipe applies its arguments in the order GIVEN, left to right, while compose applies its arguments in REVERSE, right to left — the real, standard, math-notation-inspired convention (compose mirrors mathematical function composition (f ∘ g)(x) = f(g(x))).

2. Clarifying questions a strong candidate asks

Question	Why it matters
compose direction?	Right-to-left —

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: compose() and pipe() produce the identical result for the same functions, with only their argument order reversed

Practice

Discussion

Other approaches, follow-ups and how it went in real interviews.

No comments yet. Share how you would answer it.

Question 93 of 154 in JavaScript coding

Keep your momentum going.

Mark this question solved to track your progress, then take the next one while it is fresh.

All JavaScript coding questions

Up nextMedium→

Flatten and Unflatten a Nested Object (dot notation & arrays)

Continue