Why type annotations are needed in Typescript - Test Your Understanding

Practice - 5 Tasks

1fill in blank

easy

Complete the code to declare a variable with a number type annotation.

Typescript

let age: [1] = 30;

Drag options to blanks, or click blank then click option'

Aany

Bstring

Cboolean

Dnumber

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'string' instead of 'number' causes type errors.

Omitting the type annotation can lead to implicit 'any' type.

The number type annotation tells TypeScript that age must hold a number value.

2fill in blank

medium

Complete the function parameter with the correct type annotation.

Typescript

function greet(name: [1]) { console.log(`Hello, ${name}!`); }

Drag options to blanks, or click blank then click option'

Anumber

Bobject

Cstring

Dboolean

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'number' or 'boolean' causes errors when passing text.

Not specifying a type can lead to unexpected behavior.

The parameter name should be a string because it represents text.

3fill in blank

hard

Fix the error by adding the correct return type annotation to the function.

Typescript

function add(a: number, b: number): [1] { return a + b; }

Drag options to blanks, or click blank then click option'

Anumber

Bboolean

Cstring

Dvoid

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'void' means no value is returned, causing errors here.

Using 'string' or 'boolean' does not match the actual return value.

The function returns the sum of two numbers, so the return type is number.

4fill in blank

hard

Fill both blanks to create a typed object with a name and age.

Typescript

const person: { name: [1]; age: [2] } = { name: 'Alice', age: 25 };

Drag options to blanks, or click blank then click option'

Astring

Bnumber

Cboolean

Dany

Attempts:

3 left

💡 Hint

Common Mistakes

Mixing up types causes type errors.

Using 'any' loses the benefit of type checking.

The name is text, so it uses string. The age is a number, so it uses number.

5fill in blank

hard

Fill all three blanks to type a function that returns a boolean indicating if a number is positive.

Typescript

function isPositive(num: [1]): [2] { return num [3] 0; }

Drag options to blanks, or click blank then click option'

Astring

Bboolean

C>

Dnumber

Attempts:

3 left

💡 Hint

Common Mistakes

Using wrong types for parameters or return values.

Using incorrect comparison operators.

The parameter num is a number. The function returns a boolean. The comparison operator is >.

Practice

1. Why do we use type annotations in TypeScript?
let age: number = 25;

easy

A. To tell the computer what kind of data to expect

B. To make the program run faster

C. To hide the data from other parts of the program

D. To automatically fix errors in the code

Solution

1. Step 1: Understand the purpose of type annotations

   Type annotations specify the kind of data a variable should hold, like number or string.

2. Step 2: Identify the benefit in the example

   In let age: number = 25;, the annotation number tells the computer to expect a number value.

3. Final Answer:

   To tell the computer what kind of data to expect -> Option A

4. Quick Check:

   Type annotations = tell data type [OK]

Hint: Type annotations describe data type before running code [OK]

Common Mistakes:

• Thinking type annotations speed up program execution
• Believing type annotations hide data
• Assuming type annotations fix errors automatically

2. Which of the following is the correct way to add a type annotation for a string variable in TypeScript?

easy

A. let name = string 'Alice';

B. let name == 'Alice': string;

C. let name: 'Alice' = string;

D. let name: string = 'Alice';

Solution

1. Step 1: Recall TypeScript syntax for type annotations

   The correct syntax is let variableName: type = value;.

2. Step 2: Match syntax with options

   let name: string = 'Alice'; matches this syntax: let name: string = 'Alice'; is correct.

3. Final Answer:

   let name: string = 'Alice'; -> Option D

4. Quick Check:

   Correct syntax = let name: string = 'Alice'; [OK]

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

Common Mistakes:

• Using '=' instead of ':' for type annotation
• Placing type after value
• Using double equals '==' for assignment

3. What will happen when you run this TypeScript code?

let count: number = 10;
count = 'ten';

medium

A. The code runs but count remains 10

B. The code runs and count becomes 'ten'

C. A compile-time error occurs because 'ten' is not a number

D. A runtime error occurs when assigning 'ten'

Solution

1. Step 1: Check the declared type of count

   Variable count is declared as number.

2. Step 2: Analyze the assignment of a string to a number

   Assigning 'ten' (a string) to count violates the type annotation.

3. Step 3: Understand TypeScript behavior

   TypeScript will show a compile-time error before running the code.

4. Final Answer:

   A compile-time error occurs because 'ten' is not a number -> Option C

5. Quick Check:

   Type mismatch = compile-time error [OK]

Hint: Assign values matching declared type to avoid errors [OK]

Common Mistakes:

• Thinking TypeScript allows wrong type assignments at runtime
• Confusing compile-time and runtime errors
• Assuming variable value stays unchanged silently

4. Identify the error in this TypeScript code and choose the fix:

function greet(name) {
  return 'Hello, ' + name.toUpperCase();
}
greet(123);

medium

A. Call greet with a string instead of a number

B. Add type annotation: name: string in function parameter

C. Remove the function parameter

D. Change toUpperCase() to toString()

Solution

1. Step 1: Understand the error cause

   The function expects a string to call toUpperCase(), but 123 is a number.

2. Step 2: Fix by adding type annotation

   Adding name: string tells TypeScript to expect a string and catch wrong calls like greet(123).

3. Final Answer:

   Add type annotation: name: string in function parameter -> Option B

4. Quick Check:

   Type annotation prevents wrong argument types [OK]

Hint: Add parameter types to catch wrong argument types early [OK]

Common Mistakes:

• Ignoring type annotations and relying on runtime errors
• Changing method instead of fixing parameter type
• Removing parameters instead of typing them

5. You have this TypeScript function:

function calculateArea(width, height) {
  return width * height;
}

How can adding type annotations improve this function?

hard

A. By ensuring both width and height are numbers to avoid wrong calculations

B. By making the function run faster

C. By allowing width and height to be any type

D. By automatically converting strings to numbers

Solution

1. Step 1: Identify the problem without type annotations

   Without types, width and height can be any type, causing wrong results if strings or other types are passed.

2. Step 2: Add type annotations to improve safety

   Adding width: number and height: number ensures only numbers are accepted, preventing errors.

3. Final Answer:

   By ensuring both width and height are numbers to avoid wrong calculations -> Option A

4. Quick Check:

   Type annotations improve input safety [OK]

Hint: Type inputs to avoid wrong data causing bugs [OK]

Common Mistakes:

• Thinking type annotations speed up execution
• Assuming type annotations convert data types automatically
• Allowing any type defeats purpose of annotations