Rest elements in tuples in Typescript - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to declare a tuple with a rest element that collects numbers.

Typescript

type Numbers = [string, ...[1][]];

Drag options to blanks, or click blank then click option'

Astring

Bnumber

Cboolean

Dany

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'string' instead of 'number' for the rest element.

Forgetting the spread operator '...'.

The rest element ...number[] collects all remaining elements as numbers in the tuple.

2fill in blank

medium

Complete the code to define a tuple type where the first element is a boolean and the rest are strings.

Typescript

type BoolStrings = [[1], ...string[]];

Drag options to blanks, or click blank then click option'

Aboolean

Bstring

Cnumber

Dany

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'string' or 'number' instead of 'boolean' for the first element.

Placing the rest element before the first element.

The first element is a boolean, followed by any number of strings collected by the rest element.

3fill in blank

hard

Fix the error in the tuple type that tries to use multiple rest elements.

Typescript

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

Drag options to blanks, or click blank then click option'

Aboolean

Bnumber

Cstring

Dany

Attempts:

3 left

💡 Hint

Common Mistakes

Trying to add multiple rest elements in a tuple.

Placing the rest element in the middle of the tuple.

TypeScript tuples can only have one rest element, and it must be last. The last part should be removed or corrected.

4fill in blank

hard

Fill both blanks to create a tuple type with a string first element and a rest element collecting booleans.

Typescript

type TupleWithRest = [[1], ...[2][]];

Drag options to blanks, or click blank then click option'

Astring

Bnumber

Cboolean

Dany

Attempts:

3 left

💡 Hint

Common Mistakes

Swapping the types for the first element and rest element.

Using 'number' instead of 'boolean' for the rest element.

The tuple starts with a string, followed by any number of booleans collected by the rest element.

5fill in blank

hard

Fill all three blanks to define a tuple type with a number first element, a boolean second element, and a rest element collecting strings.

Typescript

type ComplexTuple = [[1], [3], ...[2][]];

Drag options to blanks, or click blank then click option'

Anumber

Bstring

Cboolean

Dany

Attempts:

3 left

💡 Hint

Common Mistakes

Placing the rest element after the last element.

Using wrong types for the positions.

The tuple starts with a number, followed by a boolean, then any number of strings collected by the rest element.

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