Explicit type annotations in Typescript - Mini Project: Build & Apply

Explicit type annotations

📖 Scenario: You are creating a simple program to store and display information about a book in a library system.

🎯 Goal: Learn how to use explicit type annotations in TypeScript to define the types of variables and function parameters.

📋 What You'll Learn

Create variables with explicit type annotations

Define a function with typed parameters and return type

Use the variables and function to display information

💡 Why This Matters

🌍 Real World

Explicit type annotations help developers understand what kind of data variables and functions expect, reducing bugs in real-world applications like library systems.

💼 Career

Many programming jobs require writing clear, typed code to maintain large codebases and collaborate with teams effectively.

1

Create variables with explicit type annotations

Create a variable called title with type string and value "The Great Gatsby". Create a variable called year with type number and value 1925.

Typescript

# Create a variable called title with type string and value "The Great Gatsby"
# Create a variable called year with type number and value 1925

Hint

Use let variableName: type = value; to declare variables with explicit types.

2

Add a variable with explicit boolean type

Create a variable called available with type boolean and value true.

Typescript

let title: string = "The Great Gatsby";
let year: number = 1925;
# Create a variable called available with type boolean and value true

Hint

Boolean variables can be declared with type boolean and values true or false.

3

Create a function with typed parameters and return type

Create a function called getBookInfo that takes parameters title (type string), year (type number), and available (type boolean). The function should return a string combining these values in the format: "Title (Year) - Available" or "Title (Year) - Not Available" depending on the available value.

Typescript

let title: string = "The Great Gatsby";
let year: number = 1925;
let available: boolean = true;
# Create function getBookInfo with typed parameters and return type string

Hint

Use function functionName(param: type, ...): returnType { ... } to define typed functions.

4

Display the book information using the function

Use console.log to print the result of calling getBookInfo with the variables title, year, and available.

Typescript

let title: string = "The Great Gatsby";
let year: number = 1925;
let available: boolean = true;

function getBookInfo(title: string, year: number, available: boolean): string {
    return `${title} (${year}) - ${available ? "Available" : "Not Available"}`;
}

# Print the book information using console.log and getBookInfo

Hint

Call the function with the variables and print the result using console.log.

Practice

1. What is the main purpose of using explicit type annotations in TypeScript?

easy

A. To tell TypeScript exactly what type a variable or function uses

B. To make the code run faster

C. To automatically convert types at runtime

D. To remove all type checking

Solution

1. Step 1: Understand what explicit type annotations do

   Explicit type annotations specify the exact type for variables or functions in TypeScript.

2. Step 2: Identify the purpose of explicit typing

   This helps TypeScript catch mistakes and makes the code easier to understand.

3. Final Answer:

   To tell TypeScript exactly what type a variable or function uses -> Option A

4. Quick Check:

   Explicit type annotations = specify exact types [OK]

Hint: Explicit types tell TS variable types clearly [OK]

Common Mistakes:

• Thinking explicit types speed up runtime
• Confusing type annotations with type conversion
• Believing explicit types disable type checking

2. Which of the following is the correct syntax for explicitly typing a variable as a string in TypeScript?

easy

A. let name = string;

B. let name: string;

C. let name = "string";

D. let name: "string";

Solution

1. Step 1: Recall TypeScript variable type annotation syntax

   Type annotations use a colon after the variable name followed by the type.

2. Step 2: Match the correct syntax

   let name: string; uses let name: string; which is the correct way to declare a string type.

3. Final Answer:

   let name: string; -> Option B

4. Quick Check:

   Variable: type syntax = correct [OK]

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

Common Mistakes:

• Using equals sign instead of colon for type
• Putting type inside quotes incorrectly
• Assigning type as a value instead of annotation

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

let age: number = 30;
age = "thirty";
console.log(age);

medium

A. Type error at compile time

B. thirty

C. 30

D. undefined

Solution

1. Step 1: Check the variable type and assignment

   The variable age is explicitly typed as number, but assigned a string "thirty" later.

2. Step 2: Understand TypeScript's type checking

   TypeScript will raise a compile-time error because the string assignment violates the number type annotation.

3. Final Answer:

   Type error at compile time -> Option A

4. Quick Check:

   Assigning wrong type = compile error [OK]

Hint: Assign only matching types to annotated variables [OK]

Common Mistakes:

• Assuming code runs ignoring type mismatch
• Thinking TypeScript converts types automatically
• Expecting runtime error instead of compile error

4. Identify the error in this TypeScript code snippet:

function greet(name: string): void {
  console.log("Hello, " + name);
}
greet(42);

medium

A. Variable name is not declared

B. Missing return type annotation

C. Function body syntax error

D. Calling function with wrong argument type

Solution

1. Step 1: Check function parameter type

   The function greet expects a string parameter named name.

2. Step 2: Check the function call argument

   The call greet(42) passes a number instead of a string, causing a type mismatch error.

3. Final Answer:

   Calling function with wrong argument type -> Option D

4. Quick Check:

   Argument type must match parameter type [OK]

Hint: Match argument types to parameter annotations [OK]

Common Mistakes:

• Ignoring parameter type annotations
• Assuming implicit type conversion
• Missing function return type is not an error here

5. You want to create a function that takes an array of numbers and returns an object mapping each number to its square. Which explicit type annotation correctly describes the return type?

function squareMap(nums: number[]): ??? {
  const result = {};
  for (const num of nums) {
    result[num] = num * num;
  }
  return result;
}

hard

A. Map

B. { [key: string]: number }

C. Record

D. Array<number>

Solution

1. Step 1: Understand the function's return structure

   The function returns an object mapping numbers to their squares, so keys and values are numbers.

2. Step 2: Choose the best explicit return type

   TypeScript's Record<number, number> type explicitly defines an object with number keys and number values, matching the function's output.

3. Final Answer:

   Record<number, number> -> Option C

4. Quick Check:

   Use Record for typed object maps [OK]

Hint: Use Record for typed objects [OK]

Common Mistakes:

• Using string keys when keys are numbers
• Returning array instead of object
• Using index signature with wrong key type