Type annotation on variables in Typescript - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output of this TypeScript code with variable type annotations?

Consider the following TypeScript code snippet. What will be logged to the console?

Typescript

let count: number = 5;
let message: string = "Hello";
count = count + 10;
console.log(message + " " + count);

A "Hello 15"

B "Hello 5"

C TypeError at runtime

D Compilation error due to type mismatch

💡 Hint

Look at how the variables are typed and how their values change before logging.

The variable 'count' is typed as number and initialized to 5. It is then incremented by 10, making it 15. The variable 'message' is a string 'Hello'. Concatenating them with a space results in 'Hello 15'.

❓ Predict Output

intermediate

What error occurs when assigning wrong type to a typed variable?

What happens when you run this TypeScript code?

Typescript

let isActive: boolean = true;
isActive = "yes";

A No error, isActive becomes 'yes'

B Runtime error: Cannot assign string to boolean.

C Output: true

D Compilation error: Type 'string' is not assignable to type 'boolean'.

💡 Hint

Check the type of the variable and the assigned value.

TypeScript enforces types at compile time. Assigning a string to a boolean variable causes a compilation error.

🔧 Debug

advanced

Why does this TypeScript code cause a compilation error?

Identify the cause of the error in this code snippet.

Typescript

let data: number;
data = 10;
data = "twenty";

A Error because 'data' is not initialized at declaration.

B Error because 'data' is declared as number but assigned a string.

C No error, TypeScript allows changing types.

D Error because 'data' is declared with let instead of const.

💡 Hint

Look at the type annotation and the assigned values.

The variable 'data' is typed as number. Assigning a string 'twenty' violates the type annotation, causing a compilation error.

🧠 Conceptual

advanced

What is the purpose of type annotation on variables in TypeScript?

Choose the best explanation for why we use type annotations on variables.

A To allow variables to change types dynamically during runtime.

B To make the code run faster by specifying types explicitly.

C To tell the compiler what type a variable should hold, enabling type checking and preventing errors.

D To automatically convert variables to strings when needed.

💡 Hint

Think about how TypeScript helps catch mistakes before running the code.

Type annotations help the compiler check that variables are used correctly, catching errors early and improving code quality.

❓ Predict Output

expert

What is the output of this TypeScript code with union type annotation?

Analyze the following code and select the correct console output.

Typescript

let value: number | string;
value = 42;
console.log(typeof value);
value = "forty-two";
console.log(typeof value);

A "number" followed by "string"

B Compilation error due to union type usage

C "string" followed by "number"

D "number" twice

💡 Hint

Check the type of 'value' after each assignment.

The variable 'value' can hold either a number or a string. First it holds 42 (number), then 'forty-two' (string). The typeof operator reflects these types.

Practice

1. What does the type annotation let age: number; tell us about the variable age?

easy

A. It means age can hold any type of value.

B. It means age can only hold numbers.

C. It means age is a string variable.

D. It means age is a function.

Solution

1. Step 1: Understand type annotation syntax

   The colon : after age followed by number means age must hold a number value.

2. Step 2: Interpret the meaning of number

   The type number restricts the variable to only numeric values, no strings or other types allowed.

3. Final Answer:

   It means age can only hold numbers. -> Option B

4. Quick Check:

   Type annotation : number means only numbers [OK]

Hint: Colon after variable name means type follows [OK]

Common Mistakes:

• Thinking type annotation changes the value
• Confusing type annotation with assignment
• Assuming variable can hold any type without error

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

easy

A. let name string;

B. let name = string;

C. let name: string;

D. let name: "string";

Solution

1. Step 1: Recall correct syntax for type annotation

   Type annotation uses a colon : followed by the type name without quotes.

2. Step 2: Check each option

   let name: string; uses let name: string; which is correct syntax. Options B and C miss the colon or use wrong syntax. let name: "string"; uses quotes around string, which is incorrect.

3. Final Answer:

   let name: string; -> Option C

4. Quick Check:

   Use colon and type name without quotes [OK]

Hint: Use colon then type name without quotes [OK]

Common Mistakes:

• Omitting the colon between variable and type
• Putting type in quotes like a string value
• Using equals sign instead of colon for type

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

let count: number = 5;
count = 10;
console.log(count);

medium

A. 10

B. Error: Type mismatch

C. undefined

D. 5

Solution

1. Step 1: Analyze variable declaration and assignment

   count is declared as a number and initialized to 5, then reassigned to 10, which is valid.

2. Step 2: Understand console.log output

   The last assigned value is 10, so console.log(count) prints 10.

3. Final Answer:

   10 -> Option A

4. Quick Check:

   Variable holds last assigned number value [OK]

Hint: Variable prints last assigned value [OK]

Common Mistakes:

• Thinking initial value 5 is printed
• Expecting error on reassignment with same type
• Confusing output with undefined

4. Identify the error in this TypeScript code:

let isActive: boolean = "true";

medium

A. No error, code is correct.

B. Missing semicolon at the end.

C. Variable name cannot be isActive.

D. Type mismatch: string assigned to boolean.

Solution

1. Step 1: Check type annotation and assigned value

   The variable isActive is typed as boolean but assigned a string value "true".

2. Step 2: Understand TypeScript type checking

   TypeScript does not allow assigning a string to a boolean variable, causing a type mismatch error.

3. Final Answer:

   Type mismatch: string assigned to boolean. -> Option D

4. Quick Check:

   Type annotation enforces matching value types [OK]

Hint: Check assigned value matches annotated type [OK]

Common Mistakes:

• Ignoring quotes around boolean values
• Assuming string "true" is same as boolean true
• Thinking semicolon or variable name causes error

5. You want to declare a variable data that can hold either a number or a string. Which is the correct TypeScript annotation?

hard

A. let data: number | string;

B. let data: number & string;

C. let data: number, string;

D. let data: (number string);

Solution

1. Step 1: Understand union types in TypeScript

   The pipe symbol | is used to allow a variable to hold one of multiple types.

2. Step 2: Evaluate each option

   let data: number | string; uses number | string which means data can be either number or string. let data: number & string; uses & which means intersection (both types at once), invalid here. Options C and D have incorrect syntax.

3. Final Answer:

   let data: number | string; -> Option A

4. Quick Check:

   Use | for multiple allowed types [OK]

Hint: Use pipe | to allow multiple types [OK]

Common Mistakes:

• Using & instead of | for multiple types
• Separating types with comma or space
• Putting types inside parentheses without operator