What are function overloads? | Interview Question

Q: What are function overloads?

**Target Audience:** Developers who type functions that accept several argument shapes and want precise return types, explained from first principles. **Difficulty:** Medium **TypeScript version:** Checked with TypeScript 5.4.5 and `--strict`. > **How to read this doc:** Every idea is explained in plain language first, then tagged with `📌 Interview term:` so you also learn the exact words an interviewer expects. Type claims are checked by `tsc` (an `Equal ` line or a `// @ts-expect-error` line), and the **Verified** section quotes the real output in full. ## 1. Why This Even Matters — A Story First A post office has one counter but a sign above it listing what you can hand over: "Letter → you get a stamp receipt. Parcel + weight → you get a tracking number." Behind the counter is one clerk who handles everything. Customers read the sign, not the clerk's internal notes, and if you show up with something that is not on the sign — a parcel with no weight — you are turned away befo

The problem

Question presented to candidate: "What are function overloads in TypeScript? Write a function whose return type depends on how it is called, and tell me when you would use overloads instead of a union parameter or a generic."

What a strong answer should cover:

Overloads are several call signatures (bodiless declarations) followed by one implementation; callers see only the overload signatures, never the implementation signature
Resolution goes top to bottom and picks the first overload that matches, so specific signatures must come before general ones
The implementation signature must be compatible with every overload, or tsc reports TS2394
A union-typed argument matches no single overload: overloads do not combine, which is a common surprise
Alternatives: a union parameter when the return type does not depend on the input, a generic (possibly with a conditional return type) when it does in a regular way; overloads for a few distinct, irregular shapes
Where overloads appear: methods, overloaded call signatures in a type (usable with arrow functions), and the fact that ReturnType / Parameters see only the last overload

Code / implementation expected: Yes — a function with two or three overload signatures and one implementation, called in each way.

overloads

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Solution

Target Audience: Developers who type functions that accept several argument shapes and want precise return types, explained from first principles. Difficulty: Medium TypeScript version: Checked with TypeScript 5.4.5 and --strict.

How to read this doc: Every idea is explained in plain language first, then tagged with 📌 Interview term: so you also learn the exact words an interviewer expects. Type claims are checked by tsc (an Equal line or a // @ts-expect-error line), and the Verified section quotes the real output in full.

1. Why This Even Matters — A Story First

A post office has one counter but a sign above it listing what you can hand over: "Letter → you get a stamp receipt. Parcel + weight → you get a tracking number." Behind the counter is one clerk who handles everything. Customers read the sign, not the clerk's internal notes, and if you show

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9 min read · 9 sections

Run the code

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

TypeScriptOverload signatures, ordering and compatibility

Practice

TypeScriptAlternatives, overloaded call signatures and ReturnType

Practice

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