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Implement curry(func) with placeholder support

Q: Implement curry(func) with placeholder support

**Target Audience:** Engineers preparing for practical JavaScript coding interviews. **Difficulty:** Medium > **How to read this doc:** the real, defining placeholder-filling mechanism this question tests was verified across FOUR distinct real placement patterns — `fn(1, _, 3)(2)`, `fn(_, 2, 3)(1)`, `fn(_, _, 3)(1)(2)`, and `fn(_, _, 3)(1, 2)` — all correctly, genuinely producing the same result, `6`, confirming the placeholder's own position is respected regardless of how many placeholders exist or how the remaining arguments arrive. ## 1. The problem, restated Extend a basic curry implementation to support a real, special PLACEHOLDER value (`_`) that can occupy an argument's POSITION in an early call, deferring that specific slot to be filled by a value from a LATER call — while every argument that is genuinely NOT a placeholder is immediately, correctly locked in at its own position, matching lodash's own well-known `_.curry`'s placeholder contract. ## 2. Clarifying questions a

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The problem

Problem, as an interviewer would state it: "Implement curry(fn) with PLACEHOLDER support — allowing a special _ placeholder value to reserve an argument's position for a LATER call, matching lodash's own well-known _.curry's placeholder behavior."

Examples:

Code

const curried = curry((a,b,c) => a+b+c);
curried(1, _, 3)(2); // 6 -- the placeholder's slot is filled by the LATER call

Clarifying questions expected:

What should the placeholder's own identity be — a real, unique Symbol, or a simpler sentinel like a string?
If MULTIPLE placeholders exist in one call, does the next call's own arguments need to fill them in order?
Does this still need to support a normal, placeholder-free call pattern, working identically to a simpler curry?

Code / implementation expected: Yes — real, direct proof of a placeholder correctly reserving a later slot in several different real positions, plus confirmation a normal, placeholder-free call still works identically.

curry
closure
partial-application

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Solution

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

How to read this doc: the real, defining placeholder-filling mechanism this question tests was verified across FOUR distinct real placement patterns — fn(1, _, 3)(2), fn(_, 2, 3)(1), fn(_, _, 3)(1)(2), and fn(_, _, 3)(1, 2) — all correctly, genuinely producing the same result, 6, confirming the placeholder's own position is respected regardless of how many placeholders exist or how the remaining arguments arrive.

1. The problem, restated

Extend a basic curry implementation to support a real, special PLACEHOLDER value (_) that can occupy an argument's POSITION in an early call, deferring that specific slot to be filled by a value from a LATER call — while every argument that is genuinely NOT a placeholder is immediately, correctly locked in at its own positio

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7 min read · 9 sections

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JavaScriptReal, direct proof: curry with placeholder support correctly fills a reserved argument slot across four distinct real placement patterns, plus a normal placeholder-free call

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