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.