Rest elements in tuples in Typescript - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of tuple with rest element in function parameter

What is the output of this TypeScript code when calling logTuple([1, 2, 3, 4])?

Typescript

function logTuple([first, ...rest]: [number, ...number[]]) {
  console.log(first, rest.length);
}
logTuple([1, 2, 3, 4]);

A 1 3

B 1 4

C 1 0

D Error: Tuple destructuring failed

💡 Hint

Remember that the rest element collects all remaining items after the first.

The first element is 1, and the rest collects [2, 3, 4], so rest.length is 3.

❓ Predict Output

intermediate

Length of tuple with rest element

What is the length of the tuple type T = [string, ...number[]] when assigned const x: T = ['hello', 1, 2, 3]?

Typescript

type T = [string, ...number[]];
const x: T = ['hello', 1, 2, 3];
console.log(x.length);

A Error: Tuple length cannot be determined

B 3

C 1

D 4

💡 Hint

The rest element allows any number of elements after the first.

The tuple has one string followed by any number of numbers, so length is 4 here.

🔧 Debug

advanced

Identify the error with rest element in tuple type

Why does this TypeScript code cause an error?

Typescript

type T = [...number[], string];
const x: T = [1, 2, 3, 'end'];

A Cannot mix number and string in tuple

B Tuple cannot have more than 3 elements

C Rest element must be last in tuple type

D No error, code is valid

💡 Hint

Check the position of the rest element in the tuple type.

In TypeScript, rest elements in tuple types must be last. Here, the rest is first, causing an error.

🚀 Application

advanced

Function parameter with tuple rest element

Which function signature correctly accepts a tuple with a string followed by any number of booleans?

A function f([name, ...flags]: [string, ...boolean[]]) {}

B function f([name, ...flags]: [...boolean[], string]) {}

C function f([name, ...flags]: [string, boolean]) {}

D function f([name, ...flags]: [boolean, ...string[]]) {}

💡 Hint

Rest element must be last and match the type of remaining elements.

Option A correctly defines a tuple with a string first and any number of booleans after.

🧠 Conceptual

expert

Rest elements and tuple inference behavior

Given const t = [1, 2, 3] as const; and type T = [number, ...number[]];, what is the inferred type of t and does it satisfy T?

Typescript

const t = [1, 2, 3] as const;
type T = [number, ...number[]];

A <code>t</code> is readonly [1, 2, 3] but does not satisfy <code>T</code>

B <code>t</code> is readonly [1, 2, 3] and satisfies <code>T</code>

C <code>t</code> is mutable number[] and satisfies <code>T</code>

D <code>t</code> is mutable number[] but does not satisfy <code>T</code>

💡 Hint

Consider how as const affects tuple types and readonly status.

The as const makes t a readonly tuple [1, 2, 3], which matches the pattern [number, ...number[]].

Practice

1.

What is the main purpose of using rest elements in TypeScript tuples?

easy

A. To define the exact fixed length of a tuple

B. To make tuples immutable

C. To allow tuples to contain only one type of element

D. To collect multiple elements into a single tuple part

Solution

1. Step 1: Understand rest elements in tuples

   Rest elements gather multiple items into one part of a tuple, allowing flexible length.

2. Step 2: Identify the purpose

   This means rest elements help collect many elements into one part of a tuple, not fix length or types.

3. Final Answer:

   To collect multiple elements into a single tuple part -> Option D

4. Quick Check:

   Rest elements collect multiple items [OK]

Hint: Rest elements gather extra items [OK]

Common Mistakes:

• Thinking rest elements fix tuple length
• Using ... without array type
• Confusing rest elements with tuple immutability

2.

Which of the following is the correct syntax for a tuple with a rest element in TypeScript?

type MyTuple = [number, ...string[]];

easy

A. type MyTuple = [number, string...];

B. type MyTuple = [...number, string[]];

C. type MyTuple = [number, ...string[]];

D. type MyTuple = [number, ...string];

Solution

1. Step 1: Check rest element syntax

   Rest elements use three dots (...) before a type array, placed at the end.

2. Step 2: Validate each option

   type MyTuple = [number, ...string[]]; correctly uses [...string[]] after number. Others misuse syntax or placement.

3. Final Answer:

   type MyTuple = [number, ...string[]]; -> Option C

4. Quick Check:

   Rest element syntax = ...type[] at end [OK]

Hint: Rest element syntax: ...type[] at tuple end [OK]

Common Mistakes:

• Placing rest element in the middle
• Omitting brackets around rest type
• Using ...type instead of ...type[]

3.

What will be the output of the following TypeScript code?

type Tuple = [string, ...number[]];
const example: Tuple = ['start', 1, 2, 3];
console.log(example.length);

medium

A. 1

B. 4

C. Error

D. 3

Solution

1. Step 1: Understand tuple and rest element

   The tuple starts with a string, then any number of numbers via rest element.

2. Step 2: Count elements in example

   example has 'start', 1, 2, 3 which is 4 elements total.

3. Final Answer:

   4 -> Option B

4. Quick Check:

   Tuple length counts all elements including rest [OK]

Hint: Tuple length includes rest elements count [OK]

Common Mistakes:

• Counting only fixed elements ignoring rest
• Expecting error due to rest element
• Confusing tuple length with rest array length

4.

Identify the error in this tuple type declaration:

type WrongTuple = [...string, number];

medium

A. Rest element must be an array type

B. Rest element must be at the end of the tuple

C. Tuple cannot have more than two elements

D. Number type cannot be used in tuples

Solution

1. Step 1: Check rest element syntax

   Rest elements must use an array or tuple type after ..., like ...string[].

2. Step 2: Analyze given tuple

   Here, ...string uses primitive string, not array/tuple type, invalid syntax.

3. Final Answer:

   Rest element must be an array type -> Option A

4. Quick Check:

   Rest element type = array/tuple only [OK]

Hint: Rest element requires array/tuple type [OK]

Common Mistakes:

• Using ... with non-array type like ...string
• Thinking rest element can't be array type
• Confusing tuple length limits

5.

Given the tuple type type Data = [string, ...number[], boolean], which of the following assignments is valid?

A) const a: Data = ['hello', 1, 2, true];
B) const b: Data = ['hello', 42];
C) const c: Data = ['hello', 1, 2, 3, false];
D) const d: Data = ['hello', 1, false, 2];

hard

A. Only A and C are valid

B. Only B and D are valid

C. Only C is valid

D. Only D is valid

Solution

1. Step 1: Understand tuple with rest element in middle

   Type [string, ...number[], boolean] means string first, then any numbers, then boolean last.

2. Step 2: Check each assignment

   A) Ends with true (boolean), numbers in middle: ['hello',1,2,true] valid.
   B) Missing boolean after numbers, only string and number: invalid.
   C) Ends with false (boolean), numbers in middle: valid.
   D) Boolean before number (false before 2): invalid order.

3. Final Answer:

   Only A and C are valid -> Option A

4. Quick Check:

   Rest numbers must be between string and boolean [OK]

Hint: Rest numbers must be between first string and last boolean [OK]

Common Mistakes:

• Placing boolean before rest numbers
• Omitting rest numbers entirely
• Assuming rest can be anywhere