Type annotation on function parameters in Typescript - Mini Project: Build & Apply

Type annotation on function parameters

📖 Scenario: You are building a simple calculator function that adds two numbers. To make your code clear and safe, you want to specify the types of the inputs.

🎯 Goal: Create a function called add that takes two numbers as parameters and returns their sum. Use type annotations on the function parameters.

📋 What You'll Learn

Create a function named add

Add type annotations to both parameters to specify they are numbers

Return the sum of the two parameters

💡 Why This Matters

🌍 Real World

Type annotations help catch mistakes early and make your code easier to understand, especially when working with others.

💼 Career

Many programming jobs require writing clear, typed code to reduce bugs and improve maintainability.

1

Create the function skeleton

Write a function called add that takes two parameters named a and b without any type annotations yet.

Typescript

function add(a, b) {
  // Your code here
}

Hint

Start by writing the function name and parameters without types. Then return the sum of a and b.

2

Add type annotations to parameters

Add type annotations to the parameters a and b to specify they are both of type number.

Typescript

function add(a: number, b: number) {
  return a + b;
  // Your code here
}

Hint

Use : number after each parameter name to add the type annotation.

3

Add a variable to hold the result

Inside the function add, create a variable called result of type number that stores the sum of a and b. Then return result.

Typescript

function add(a: number, b: number) {
  const result: number = a + b;
  // Your code here
  return result;
}

Hint

Declare result with const and add the type : number. Then return it.

4

Test the function output

Write a console.log statement to call add(5, 7) and print the result.

Typescript

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

// Your code here

Hint

Use console.log(add(5, 7)) to see the sum printed.

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