Type annotation on function parameters in Typescript - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output of this function call?

Consider this TypeScript function with parameter type annotations. What will be printed when calling greet('Alice')?

Typescript

function greet(name: string): string {
  return `Hello, ${name}!`;
}

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

A Error: Missing return type

B "Hello, Alice!"

C Hello, Alice!

D undefined

💡 Hint

Look at how the function returns a string and how console.log prints it.

The function returns a string with the name inserted. console.log prints the string without quotes.

❓ Predict Output

intermediate

What error does this code produce?

What error will this TypeScript code cause when compiled?

Typescript

function multiply(x: number, y: number): number {
  return x * y;
}

multiply('2', 3);

A TypeError at runtime

B No error, returns 6

C SyntaxError

D TypeScript compile error: Argument of type 'string' is not assignable to parameter of type 'number'

💡 Hint

Check the types of arguments passed to the function.

The function expects numbers, but a string is passed, causing a compile-time type error.

🔧 Debug

advanced

Why does this function cause a compile error?

This function is intended to add two numbers. Why does TypeScript report an error?

Typescript

function add(a, b: number): number {
  return a + b;
}

A Parameter 'b' should be optional

B Parameter 'a' has no type annotation, causing an implicit any error

C Return type is missing

D Function body is missing a return statement

💡 Hint

Check the types of all parameters.

Parameter 'a' lacks a type annotation, so TypeScript treats it as 'any' which may be disallowed depending on settings.

❓ Predict Output

advanced

What is the output of this function with default parameter and type annotation?

What will this code print?

Typescript

function welcome(name: string = 'Guest'): string {
  return `Welcome, ${name}!`;
}

console.log(welcome());

A Welcome, Guest!

B Welcome, undefined!

C Error: Missing argument

D Welcome, !

💡 Hint

Look at the default value for the parameter.

The parameter has a default value 'Guest', so calling without arguments uses that.

❓ Predict Output

expert

What is the output of this function using union types in parameters?

What will this code print when calling formatId(123) and formatId('abc')?

Typescript

function formatId(id: number | string): string {
  if (typeof id === 'number') {
    return `ID-${id.toFixed(0)}`;
  } else {
    return `ID-${id.toUpperCase()}`;
  }
}

console.log(formatId(123));
console.log(formatId('abc'));

A

ID-123
ID-ABC

B

ID-123.0
ID-abc

C

ID-123
ID-abc

D Error: toFixed not available on number

💡 Hint

Check how the function handles number and string types differently.

For number, toFixed(0) converts to string without decimals; for string, toUpperCase converts to uppercase.