Rest parameters with types in Typescript - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of function using typed rest parameters

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

Typescript

function sumAll(...nums: number[]): number {
  return nums.reduce((a, b) => a + b, 0);
}
console.log(sumAll(1, 2, 3, 4));

A TypeError at runtime

B 10

C undefined

D 1234

💡 Hint

The rest parameter collects all arguments into an array of numbers.

The function sums all numbers passed as arguments using reduce. The output is the sum 1+2+3+4 = 10.

❓ Predict Output

intermediate

Output of function with mixed typed rest parameters

What will this TypeScript code output when calling concatStrings('a', 'b', 'c')?

Typescript

function concatStrings(separator: string, ...strings: string[]): string {
  return strings.join(separator);
}
console.log(concatStrings('-', 'a', 'b', 'c'));

A "a-b-c"

B "-abc"

C "abc-"

D TypeError at runtime

💡 Hint

The first argument is the separator, the rest are joined with it.

The function uses the first argument as separator and joins the rest strings with it, producing 'a-b-c'.

🔧 Debug

advanced

Identify the error with rest parameter typing

What error does this TypeScript code produce?

Typescript

function multiplyAll(...nums: number): number {
  return nums.reduce((a, b) => a * b, 1);
}

A TypeError: nums.reduce is not a function

B Runtime error: missing initial value in reduce

C No error, outputs product of numbers

D SyntaxError: Rest parameter must be an array type

💡 Hint

Check the type of the rest parameter declaration.

Rest parameters must be typed as arrays (e.g., number[]). Using just 'number' is invalid syntax.

❓ Predict Output

advanced

Output of function with tuple typed rest parameters

What is the output of this TypeScript code?

Typescript

function formatName(first: string, last: string, ...titles: string[]): string {
  return `${first} ${last} ${titles.join(' ')}`.trim();
}
console.log(formatName('John', 'Doe', 'PhD', 'MD'));

A "John Doe PhD MD"

B "John Doe"

C "John Doe PhD, MD"

D TypeError at runtime

💡 Hint

The rest parameter collects all extra titles as an array.

The function concatenates first and last names with all titles joined by spaces, resulting in 'John Doe PhD MD'.

🧠 Conceptual

expert

TypeScript rest parameter with tuple types

Given this function signature, what is the type of the rest parameter args?

function example(...args: [number, string, boolean]): void;

A An array of numbers only

B An array of any length containing number, string, or boolean in any order

C A tuple with exactly three elements: number, string, boolean

D A single argument of type array containing number, string, boolean

💡 Hint

Tuple types specify fixed length and types in order.

The rest parameter typed as a tuple means exactly three arguments must be passed: number, string, boolean in that order.

Practice

1. What does the rest parameter syntax ...args: number[] do in a TypeScript function?

easy

A. It collects all remaining arguments into an array of numbers.

B. It restricts the function to accept only one number argument.

C. It declares a single number variable named args.

D. It spreads an array of numbers into separate arguments.

Solution

1. Step 1: Understand rest parameter syntax

   The ...args gathers all extra arguments passed to the function into one array.

2. Step 2: Recognize the type annotation

   The : number[] means this array contains only numbers.

3. Final Answer:

   It collects all remaining arguments into an array of numbers. -> Option A

4. Quick Check:

   Rest parameter with type = collects arguments as typed array [OK]

Hint: Rest parameters gather extra arguments into a typed array [OK]

Common Mistakes:

• Confusing rest parameter with spread operator
• Thinking it accepts only one argument
• Ignoring the type annotation on the rest parameter

2. Which of the following is the correct syntax to declare a function with a rest parameter of strings in TypeScript?

easy

A. function greet(...names: string[]) {}

B. function greet(names: ...string[]) {}

C. function greet(...names: string) {}

D. function greet(...names: [string]) {}

Solution

1. Step 1: Identify correct rest parameter syntax

   The rest parameter must use ... before the parameter name and be typed as an array type.

2. Step 2: Check each option

   function greet(...names: string[]) {} uses ...names: string[], which is correct. Others misuse the spread or array syntax.

3. Final Answer:

   function greet(...names: string[]) {} -> Option A

4. Quick Check:

   Rest parameter syntax = ...name: type[] [OK]

Hint: Rest parameter needs ... before name and array type after [OK]

Common Mistakes:

• Placing ... after the parameter name
• Using a single type instead of an array type
• Using tuple syntax instead of array type

3. What is the output of this TypeScript code?

function sum(...nums: number[]) {
  return nums.reduce((a, b) => a + b, 0);
}
console.log(sum(1, 2, 3, 4));

medium

A. 1234

B. Error: nums is not iterable

C. undefined

D. 10

Solution

1. Step 1: Understand rest parameter collects arguments

   The ...nums collects all arguments into an array: [1, 2, 3, 4].

2. Step 2: Calculate sum using reduce

   The reduce adds all numbers: 1+2=3, 3+3=6, 6+4=10.

3. Final Answer:

   10 -> Option D

4. Quick Check:

   Sum of 1,2,3,4 = 10 [OK]

Hint: Rest gathers numbers; reduce sums them [OK]

Common Mistakes:

• Concatenating numbers as strings
• Forgetting initial value in reduce
• Misunderstanding rest parameter as single number

4. Identify the error in this TypeScript function:

function joinStrings(...strings: string) {
  return strings.join(", ");
}

medium

A. The function should return an array, not a string.

B. The join method cannot be used on strings.

C. Rest parameter must be typed as an array, e.g., string[].

D. Rest parameter cannot be used with strings.

Solution

1. Step 1: Check rest parameter type

   The rest parameter must be typed as an array type, like string[], not just string.

2. Step 2: Understand why join fails

   Since strings is typed as a single string, join method is invalid.

3. Final Answer:

   Rest parameter must be typed as an array, e.g., string[]. -> Option C

4. Quick Check:

   Rest parameter type = array type [OK]

Hint: Rest parameter type must be an array type [OK]

Common Mistakes:

• Typing rest parameter as a single element type
• Trying to call array methods on a non-array
• Ignoring TypeScript type errors

5. You want to create a TypeScript function multiplyAll that accepts any number of numeric arguments and returns their product. Which is the best way to type the rest parameter and implement this function?

hard

A. function multiplyAll(nums: number[]): number { return nums.reduce((a, b) => a * b); }

B. function multiplyAll(...nums: number[]): number { return nums.reduce((a, b) => a * b, 1); }

C. function multiplyAll(...nums: number): number { return nums.reduce((a, b) => a * b, 1); }

D. function multiplyAll(...nums: any[]): any { return nums.reduce((a, b) => a * b, 0); }

Solution

1. Step 1: Choose correct rest parameter typing

   The rest parameter must be typed as number[] to accept multiple numbers.

2. Step 2: Implement product with correct initial value

   Use reduce with initial value 1 to multiply all numbers correctly.

3. Step 3: Evaluate options

   function multiplyAll(...nums: number[]): number { return nums.reduce((a, b) => a * b, 1); } matches correct typing and logic. function multiplyAll(nums: number[]): number { return nums.reduce((a, b) => a * b); } lacks rest parameter. function multiplyAll(...nums: number): number { return nums.reduce((a, b) => a * b, 1); } has wrong type. function multiplyAll(...nums: any[]): any { return nums.reduce((a, b) => a * b, 0); } uses any and wrong initial value.

4. Final Answer:

   function multiplyAll(...nums: number[]): number { return nums.reduce((a, b) => a * b, 1); } -> Option B

5. Quick Check:

   Rest parameter typed number[] and reduce with 1 = correct [OK]

Hint: Rest parameter as number[] and reduce with 1 for product [OK]

Common Mistakes:

• Using single number type instead of array for rest
• Forgetting initial value in reduce
• Using any type losing type safety
• Starting reduce with 0 causing product always 0