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.

Practice

1. What does the type annotation in the function parameter function greet(name: string) tell us?

easy

A. The function expects name to be a string.

B. The function will return a string.

C. The function does not accept any parameters.

D. The function expects name to be a number.

Solution

1. Step 1: Understand the syntax of type annotation

   The colon : after name means we are telling TypeScript what type name should be.

2. Step 2: Identify the type specified

   The type specified is string, so name must be a string.

3. Final Answer:

   The function expects name to be a string. -> Option A

4. Quick Check:

   Parameter type annotation = string [OK]

Hint: Look for colon after parameter name to find its type [OK]

Common Mistakes:

• Confusing parameter type with return type
• Ignoring the colon and thinking it's optional
• Assuming no parameters are accepted

2. Which of the following is the correct way to add a type annotation for a number parameter age in a TypeScript function?

easy

A. function setAge(age = number) {}

B. function setAge(age: number) {}

C. function setAge(number age) {}

D. function setAge(age) : number {}

Solution

1. Step 1: Recall correct syntax for parameter type annotation

   TypeScript uses a colon : after the parameter name to specify its type.

2. Step 2: Check each option

   function setAge(age: number) {} uses age: number, which is correct. Others use invalid syntax.

3. Final Answer:

   function setAge(age: number) {} -> Option B

4. Quick Check:

   Parameter type syntax = colon after name [OK]

Hint: Use colon after parameter name for type [OK]

Common Mistakes:

• Using equals sign instead of colon for type
• Placing type before parameter name
• Adding colon after parentheses instead of parameter

3. What will be the output of this TypeScript code?

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

console.log(multiply(3, 4));

medium

A. 34

B. Error: Missing return type

C. undefined

D. 12

Solution

1. Step 1: Understand function parameters and return

   The function takes two numbers and returns their product.

2. Step 2: Calculate multiply(3, 4)

   3 times 4 equals 12, so the function returns 12.

3. Final Answer:

   12 -> Option D

4. Quick Check:

   3 * 4 = 12 [OK]

Hint: Multiply parameters as numbers to get output [OK]

Common Mistakes:

• Confusing string concatenation with multiplication
• Ignoring return type annotation
• Expecting error due to missing type

4. Identify the error in this TypeScript function:

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

medium

A. Incorrect use of backticks for string

B. Missing return type annotation

C. Missing colon between parameter name and type

D. Parameter name cannot be 'name'

Solution

1. Step 1: Check parameter syntax

   The parameter name string is missing a colon : between name and string.

2. Step 2: Confirm other parts are correct

   Backticks and parameter name are valid; return type annotation is optional.

3. Final Answer:

   Missing colon between parameter name and type -> Option C

4. Quick Check:

   Parameter type needs colon [OK]

Hint: Always put colon between parameter and type [OK]

Common Mistakes:

• Omitting colon between parameter and type
• Thinking return type is mandatory
• Confusing string quotes with backticks

5. You want to write a function formatPrice that takes a number price and returns a string with a dollar sign. Which is the correct way to add type annotations to the parameter and return type?

hard

A. function formatPrice(price: number): string { return `$${price}`; }

B. function formatPrice(price): string { return `$${price}`; }

C. function formatPrice(price: string): number { return `$${price}`; }

D. function formatPrice(price: number) { return price; }

Solution

1. Step 1: Determine parameter type

   The parameter price should be a number because it represents a price value.

2. Step 2: Determine return type

   The function returns a string with a dollar sign, so return type is string.

3. Step 3: Check options

   function formatPrice(price: number): string { return `$${price}`; } correctly annotates price: number and return type : string. Others have missing or wrong types.

4. Final Answer:

   function formatPrice(price: number): string { return `$${price}`; } -> Option A

5. Quick Check:

   Parameter number, return string [OK]

Hint: Parameter type matches input, return type matches output [OK]

Common Mistakes:

• Omitting parameter type
• Wrong return type annotation
• Returning wrong data type