Why TypeScript over JavaScript - Test Your Understanding

Practice - 5 Tasks

1fill in blank

easy

Complete the code to declare a variable with a type in TypeScript.

Typescript

let age: [1] = 25;

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

Avar

Blet

Cstring

Dnumber

Attempts:

3 left

💡 Hint

Common Mistakes

Using JavaScript keywords like 'var' or 'let' as types.

Using 'string' when the value is a number.

In TypeScript, you specify the type after the colon. Here, number is the correct type for the variable age.

2fill in blank

medium

Complete the code to define a function parameter type in TypeScript.

Typescript

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

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

Anumber

Bstring

Cboolean

Dany

Attempts:

3 left

💡 Hint

Common Mistakes

Using number or boolean for text data.

Using any without reason.

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

3fill in blank

hard

Fix the error by completing the type annotation for the array.

Typescript

let scores: [1] = [10, 20, 30];

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

Anumber[]

Bany

Cboolean[]

Dstring[]

Attempts:

3 left

💡 Hint

Common Mistakes

Using string[] when the array has numbers.

Using boolean[] incorrectly.

The array scores contains numbers, so its type is number[].

4fill in blank

hard

Fill in the blank to create a typed object in TypeScript.

Typescript

let person: [1] = { name: 'Alice', age: 30 };

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

Aany

B{ name: number; age: string }

C{ name: string; age: number }

Dobject

Attempts:

3 left

💡 Hint

Common Mistakes

Swapping the types of name and age.

Using any or object without specific types.

The object person has a name as a string and age as a number, so the type annotation matches that structure.

5fill in blank

hard

Fill all three blanks to create a typed function with parameters and return type.

Typescript

function add(a: [1], b: [2]): [3] { return a + b; }

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

Astring

Bnumber

Dboolean

Attempts:

3 left

💡 Hint

Common Mistakes

Using string types for parameters or return value.

Using boolean as a type here.

The function add takes two numbers and returns a number, so the parameter types and return type are number.

Practice

1. Why do many developers choose TypeScript over plain JavaScript?

easy

A. Because TypeScript uses fewer lines of code than JavaScript

B. Because TypeScript runs faster in browsers than JavaScript

C. Because TypeScript does not require any compilation

D. Because TypeScript helps catch errors before running the code

Solution

1. Step 1: Understand TypeScript's main benefit

   TypeScript adds types to JavaScript, which helps find mistakes early, before running the code.

2. Step 2: Compare options with this benefit

   Only Because TypeScript helps catch errors before running the code mentions catching errors early, which is the key reason developers prefer TypeScript.

3. Final Answer:

   Because TypeScript helps catch errors before running the code -> Option D

4. Quick Check:

   Type safety = Catch errors early [OK]

Hint: Remember: TypeScript finds bugs before running code [OK]

Common Mistakes:

• Thinking TypeScript runs faster in browsers
• Believing TypeScript needs no compilation
• Assuming TypeScript always reduces code length

2. Which of the following is the correct way to declare a typed variable in TypeScript?

easy

A. let age = number 25;

B. let age: number = 25;

C. let age = 25: number;

D. let age number = 25;

Solution

1. Step 1: Recall TypeScript variable syntax

   In TypeScript, the type comes after the variable name, separated by a colon, like let varName: type.

2. Step 2: Check each option's syntax

   Only let age: number = 25; uses the correct syntax: let age: number = 25;. Others have misplaced or missing colons.

3. Final Answer:

   let age: number = 25; -> Option B

4. Quick Check:

   Variable: type = value [OK]

Hint: Type comes after variable name with colon [OK]

Common Mistakes:

• Placing type before variable name
• Using colon after value instead of after variable
• Omitting colon between variable and type

3. What will be the output of this TypeScript code after compilation and running in JavaScript?

function greet(name: string) {
  return `Hello, ${name.toUpperCase()}!`;
}
console.log(greet("world"));

medium

A. Hello, WORLD!

B. Error: name.toUpperCase is not a function

C. HELLO, WORLD!

D. Hello, world!

Solution

1. Step 1: Understand the function behavior

   The function takes a string and returns "Hello, " plus the uppercase version of the name plus "!".

2. Step 2: Apply the function to input "world"

   Calling name.toUpperCase() on "world" returns "WORLD", so the output is "Hello, WORLD!".

3. Final Answer:

   Hello, WORLD! -> Option A

4. Quick Check:

   toUpperCase("world") = "WORLD" [OK]

Hint: Remember string methods like toUpperCase() work on strings [OK]

Common Mistakes:

• Ignoring toUpperCase effect
• Assuming output is all uppercase
• Expecting runtime error due to types

4. Identify the error in this TypeScript code snippet:

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

console.log(add(5, "10"));

medium

A. Type error: second argument should be a number, not a string

B. No error, outputs 15

C. Syntax error: missing return type

D. Runtime error: cannot add number and string

Solution

1. Step 1: Check function parameter types

   The function expects both parameters to be numbers.

2. Step 2: Analyze the call with arguments (5, "10")

   The second argument is a string, which conflicts with the expected number type, causing a TypeScript type error before running.

3. Final Answer:

   Type error: second argument should be a number, not a string -> Option A

4. Quick Check:

   Type mismatch argument = Error [OK]

Hint: Check argument types match parameter types [OK]

Common Mistakes:

• Assuming implicit string to number conversion
• Thinking missing return type causes error
• Expecting runtime error instead of compile-time

5. You want to create a function in TypeScript that accepts either a string or a number and returns its string length or digit count. Which approach best uses TypeScript's features to handle this safely?

hard

A. Declare two separate functions, one for string and one for number

B. Declare the parameter as type any and convert it to string inside

C. Use a union type for the parameter and check its type inside the function

D. Use JavaScript without types to avoid complexity

Solution

1. Step 1: Understand TypeScript union types

   Union types allow a variable to be one of several types, e.g., string | number.

2. Step 2: Use type checking inside function

   Inside the function, use typeof to check if the input is string or number, then handle accordingly. This keeps type safety and clarity.

3. Final Answer:

   Use a union type for the parameter and check its type inside the function -> Option C

4. Quick Check:

   Union types + type guards = safe handling [OK]

Hint: Use union types and type checks for multiple input types [OK]

Common Mistakes:

• Using any type loses type safety
• Creating duplicate functions unnecessarily
• Avoiding TypeScript features by using plain JavaScript