Variadic Tuple Types

A variadic tuple type places a spread element inside a tuple — like [first, ...rest] or [...T, last] — letting its length and element types vary with a type parameter so functions such as concat and curry stay precisely typed.

Learn Variadic Tuple Types in our free TypeScript course — an interactive lesson with runnable examples, a practice exercise and a quick reference.

Part of the free TypeScript course at LearnCodingFast — hands-on lessons with examples you run in your browser, plus practice exercises and a quick quiz.

What You'll Learn

💡 Real-World Analogy

Think of a train. A plain array is "some carriages" — you've forgotten which is the engine and which is the caboose. A variadic tuple is the precise consist: "engine, then these middle carriages, then the caboose." [first, ...rest] says "engine plus whatever follows"; [...rest, last] says "whatever comes before, then the caboose." Every carriage keeps its exact role and position, no matter how many sit in the middle.

1. Spreads Inside Tuple Types

A spread element inside a tuple type makes it variadic : [id: number, ...rest: T] is "a number, then however many elements T describes." The spread can sit at the start, middle, or end, mirroring how the array spread operator works at runtime — though a tuple type may contain at most one rest element.

2. Typing concat & Labeled Elements

The headline use is a concat whose return type is [...T, ...U] : it joins two tuples while keeping every element's type and position, so result[0] stays a number . Labeled tuple elements like [x: number, y: number] add names for readability — type-only, never at runtime.

3. Rest Parameters as Tuples & Currying

TypeScript types a function's ...args as a tuple , so variadic tuples are what make bind , currying , and partial application type-safe. Splitting a parameter tuple into a head and tail — [...H, ...T] — lets you fix some arguments now and supply the rest later, each part fully typed.

🎯 Your Turn

Build the runtime behind prepend<H, T>(head, tail): [H, ...T] — put head first, then spread the rest. Fill in the two blanks marked ___ , then run it.

Common Errors (and the fix)

• ❌ "A rest element must be last in a tuple type" in old TS — you put a spread before fixed elements. ✅ Modern TS allows leading/middle spreads; ensure you're on TS 4.0+.
• ❌ "A tuple type cannot be indexed with..." — you indexed past a variadic part's known positions. ✅ Only fixed positions have a precise type; the spread region is element-typed.
• ❌ Two rest elements — [...string[], ...number[]] is invalid. ✅ A tuple type may contain at most one rest element.
• ❌ Result widens to an array — you annotated the return as T[] . ✅ Return the variadic tuple [...T, ...U] to keep positions.

Pro Tips

• 💡 Label your tuple slots — [x: number, y: number] reads far better than two anonymous numbers.
• 💡 Combine with Parameters<F> to build typed pipe , compose , and partial utilities.
• 💡 Constrain spreads with T extends unknown[] so callers must pass an array-like tuple.
• 💡 Reach for variadic tuples whenever an array's positions matter — they're how you avoid losing type precision through a helper.

📚 Keep Learning

• 📖 TS 4.0 Release Notes — Variadic Tuple Types
• 📖 TS 4.0 — Labeled Tuple Elements
• 📖 Handbook — Tuple Types

Frequently Asked Questions

Mini-Challenge: pair() and swap()

No blanks this time — just a brief and a starting outline. Build the tuple-preserving helpers yourself, run them, and check your output against the example in the comments.

🎉 Lesson Complete

• ✅ A variadic tuple type spreads a type parameter inside a tuple
• ✅ [first, ...rest] and [...rest, last] prepend and append precisely
• ✅ A concat returning [...T, ...U] keeps every element's type and position
• ✅ Labeled tuple elements ( [x: number, y: number] ) document slots — type-only
• ✅ Rest parameters are tuples , which is how curry / partial get typed
• ✅ Next lesson: Advanced Utility Types

Practice quiz

Continue this course

• Previous: Function Types & this
• Next: Advanced Utility Types

Related lessons

• Tuple Types