What are tuple types? | Interview Question

Q: What are tuple types?

**Target Audience:** Beginners who know arrays and want to describe fixed-shape data such as pairs and records. **Difficulty:** Easy **TypeScript version:** Every compiler message and output below was produced with TypeScript 5.4.5 (`--strict`, target ES2022). > **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. All output shown was produced by actually running the code; the **Verified** section quotes it in full. ## 1. Why This Even Matters — A Story First An egg carton holds any number of eggs, all of the same kind. That is an **array**: `number[]` says "some numbers, how many we do not know". A bento box is different. It has exactly three compartments, and each one has a job: rice on the left, fish in the middle, pickles on the right. Put fish where the rice goes and anyone can see something is wrong. That is a **tuple**: a fixed number of slots, each with its

The problem

Question presented to candidate: "What is a tuple type in TypeScript, how is it different from an ordinary array, and where have you used one?"

What a strong answer should cover:

A tuple fixes the length and the type at each position, such as [string, number]; its length is a literal type
How it differs from (string | number)[], and that an array literal is inferred as an array unless you annotate it or use as const
Optional elements ([number, number?]), rest elements ([string, ...string[]], also leading or middle since TypeScript 4.2) and labels ([x: number, y: number], TypeScript 4.0)
Tuples as rest parameters and variadic tuple types ([...T]) for typing argument lists
Everyday uses: returning a pair like useState, Object.entries style key/value pairs, coordinates
The pitfall: a mutable tuple still has push, so prefer readonly tuples for values you return

Code / implementation expected: Optional — declare a tuple, destructure it, and show one rejected out-of-range access.

tuple

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Solution

Target Audience: Beginners who know arrays and want to describe fixed-shape data such as pairs and records. Difficulty: Easy TypeScript version: Every compiler message and output below was produced with TypeScript 5.4.5 (--strict, target ES2022).

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. All output shown was produced by actually running the code; the Verified section quotes it in full.

1. Why This Even Matters — A Story First

An egg carton holds any number of eggs, all of the same kind. That is an array: number[] says "some numbers, how many we do not know".

A bento box is different. It has exactly three compartments, and each one has a job: rice on the left, fish in the middle, pickles on the right. Put fish where the rice goes

The solution is hidden so you can try first

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.

TypeScriptTuple basics: positions, length and rejected access

Practice

TypeScriptOptional, rest, labelled and variadic tuples

Practice

Discussion

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