Parameters type in Typescript - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of function with typed parameters

What is the output of this TypeScript code when calling greet('Alice', 3)?

Typescript

function greet(name: string, times: number): string {
  return `${name} `.repeat(times).trim();
}

console.log(greet('Alice', 3));

A "Alice Alice Alice"

B "Alice3"

C "Alice Alice"

D "3 Alice"

💡 Hint

Look at how the repeat method works on strings.

The function repeats the string "Alice " three times, resulting in "Alice Alice Alice ". The trim() removes the trailing space, so the output is "Alice Alice Alice".

❓ Predict Output

intermediate

Type error with wrong parameter type

What error will TypeScript show for this code?

Typescript

function multiply(a: number, b: number): number {
  return a * b;
}

multiply(5, '2');

A Type error: Argument of type 'string' is not assignable to parameter of type 'number'.

B Runtime error: Cannot multiply number and string.

C No error, output is 10.

D Syntax error: Missing semicolon.

💡 Hint

Check the types of arguments passed to the function.

The function expects both parameters to be numbers. Passing a string causes a compile-time type error in TypeScript.

🔧 Debug

advanced

Why does this function cause a type error?

Identify the cause of the type error in this code snippet.

Typescript

function formatDate(date: Date, format: string = 'YYYY-MM-DD'): string {
  return date.toLocaleDateString(format);
}

formatDate(new Date());

A The <code>Date</code> type is not imported.

B The function is missing a return type annotation.

C The <code>toLocaleDateString</code> method does not accept a string format parameter.

D The default parameter value is invalid syntax.

💡 Hint

Check the official signature of toLocaleDateString.

The toLocaleDateString method accepts optional locales and options objects, not a string format. Passing a string causes a type error.

📝 Syntax

advanced

Correct parameter type for callback function

Which option correctly types the parameter of this callback function?

Typescript

function processItems(items: string[], callback: (item: string) => void) {
  items.forEach(callback);
}

processItems(['a', 'b'], (item) => console.log(item));

A (item: number) => void

B (item: string) => void

C (item: any) => string

D (item) => number

💡 Hint

The array contains strings, so the callback parameter should match that type.

The callback receives each string item and returns nothing (void). Option B matches this signature exactly.

🚀 Application

expert

Determine the type of parameters in a generic function

Given this generic function, what is the type of parameter value when calling identity(42)?

Typescript

function identity<T>(value: T): T {
  return value;
}

const result = identity(42);

A unknown

B string

C any

D number

💡 Hint

TypeScript infers the type from the argument passed.

When calling identity(42), TypeScript infers T as number, so value is of type number.