Creating type aliases in Typescript - Practice Exercises

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of a type alias with union types

What is the output of the following TypeScript code when compiled and run with console.log?

Typescript

type Status = "success" | "error";

function printStatus(status: Status) {
  if (status === "success") {
    console.log("Operation succeeded");
  } else {
    console.log("Operation failed");
  }
}

printStatus("success");
printStatus("error");

A Type error at compile time

B Operation succeeded\nOperation failed

C success\nerror

D Operation failed\nOperation succeeded

💡 Hint

Check how the function uses the union type to decide what to print.

The type alias 'Status' allows only 'success' or 'error'. The function prints a message based on the value. Calling with 'success' prints 'Operation succeeded', and with 'error' prints 'Operation failed'.

🧠 Conceptual

intermediate

Understanding type alias usage

Which of the following best describes the purpose of a type alias in TypeScript?

A It creates a new variable that holds a value of a specific type.

B It imports types from external modules.

C It declares a function that returns a specific type.

D It defines a new name for an existing type or combination of types.

💡 Hint

Think about how type aliases help with readability and reuse.

Type aliases let you create a new name for an existing type or a combination of types, making code easier to read and maintain.

🔧 Debug

advanced

Identify the error in type alias usage

What error will this TypeScript code produce?

Typescript

type Point = { x: number; y: number };

const p: Point = { x: 10 };
console.log(p);

A Error: Property 'y' is missing in type '{ x: number; }' but required in type 'Point'.

B No error, outputs: { x: 10 }

C Error: Type 'number' is not assignable to type 'string'.

D Error: Cannot redeclare block-scoped variable 'p'.

💡 Hint

Check if all required properties of the type alias are present in the object.

The type alias 'Point' requires both 'x' and 'y' properties. The object assigned to 'p' only has 'x', so TypeScript reports a missing property error.

📝 Syntax

advanced

Correct syntax for type alias with intersection

Which option correctly defines a type alias combining two types using intersection?

A type Combined = TypeA | TypeB;

B type Combined = TypeA + TypeB;

C type Combined = TypeA & TypeB;

D type Combined = TypeA && TypeB;

💡 Hint

Intersection means the type must have all properties from both types.

The '&' symbol is used in TypeScript to create an intersection type alias combining all properties from both types.

🚀 Application

expert

Determine the type of a complex alias

Given the following type alias, what is the type of variable data?

Typescript

type Response<T> = {
  status: "ok" | "error";
  payload: T;
};

const data: Response<string[]> = {
  status: "ok",
  payload: ["apple", "banana"]
};

A An object with a status string and a payload array of strings.

B An object with a status number and a payload string.

C An array of strings.

D A string representing the status.

💡 Hint

Look at how the generic type parameter T is used in the alias.

The type alias 'Response' creates an object with a 'status' property that can be 'ok' or 'error', and a 'payload' property of type T. Here, T is string[], so payload is an array of strings.

Practice

1. What is the main purpose of creating a type alias in TypeScript?

easy

A. To import external modules

B. To create a new variable

C. To define a function

D. To give a new name to a type for easier reuse

Solution

1. Step 1: Understand what a type alias does

   A type alias creates a new name for an existing type, making code easier to read and reuse.

2. Step 2: Compare options with the definition

   Only To give a new name to a type for easier reuse correctly describes this purpose; others describe unrelated concepts.

3. Final Answer:

   To give a new name to a type for easier reuse -> Option D

4. Quick Check:

   Type alias = new name for type [OK]

Hint: Type aliases rename types to simplify code [OK]

Common Mistakes:

• Confusing type alias with variable declaration
• Thinking type alias creates new data
• Mixing type alias with function definitions

2. Which of the following is the correct syntax to create a type alias named Age for the type number?

easy

A. alias Age: number;

B. type Age = number;

C. let Age: number;

D. type Age == number;

Solution

1. Step 1: Recall TypeScript syntax for type aliases

   The correct syntax uses the keyword type, followed by the alias name, an equals sign, and the type.

2. Step 2: Check each option

   type Age = number; matches the correct syntax exactly. Options B, C, and D have syntax errors or wrong keywords.

3. Final Answer:

   type Age = number; -> Option B

4. Quick Check:

   type alias syntax = type Name = Type [OK]

Hint: Use 'type Name = Type;' to create aliases [OK]

Common Mistakes:

• Using 'alias' instead of 'type'
• Using '==' instead of '='
• Confusing type alias with variable declaration

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

type ID = string | number;
function printId(id: ID) {
  console.log(typeof id);
}
printId(123);
printId('abc');

medium

A. Error at compile time

B. string
number

C. number
string

D. ID
ID

Solution

1. Step 1: Understand the type alias and function behavior

   The alias ID allows string or number. The function prints the type of the argument.

2. Step 2: Trace the function calls

   Calling printId(123) prints 'number'. Calling printId('abc') prints 'string'.

3. Final Answer:

   number string -> Option C

4. Quick Check:

   typeof 123 = number, typeof 'abc' = string [OK]

Hint: Check typeof values for union type inputs [OK]

Common Mistakes:

• Assuming type alias changes runtime behavior
• Expecting compile error for union types
• Mixing order of printed types

4. Identify the error in this TypeScript code:

type User = {
  name: string;
  age: number;
}

const user: User = {
  name: 'Alice',
  age: '25'
};

medium

A. Property 'age' should be a number, not a string

B. Missing semicolon after type alias

C. Type alias name cannot be 'User'

D. Object properties must be declared with 'let'

Solution

1. Step 1: Check the type alias definition

   The alias User expects age to be a number.

2. Step 2: Check the object assigned to user

   The object has age: '25', which is a string, causing a type error.

3. Final Answer:

   Property 'age' should be a number, not a string -> Option A

4. Quick Check:

   Type mismatch in object property [OK]

Hint: Match property types exactly in objects [OK]

Common Mistakes:

• Ignoring type mismatch in object properties
• Thinking semicolon is mandatory after type alias
• Confusing variable declaration rules with type alias

5. Given these type aliases:

type Point = { x: number; y: number };
type Circle = { center: Point; radius: number };
type Shape = Circle | Point;

Which of the following is a valid Shape object?

hard

A. { center: { x: 0, y: 0 }, radius: 5 }

B. { x: 0, y: 0, radius: 5 }

C. { center: { x: 0 }, radius: 5 }

D. { x: '0', y: 0 }

Solution

1. Step 1: Understand the type aliases

   Point is an object with x and y as numbers. Circle has a center of type Point and a radius number. Shape can be either.

2. Step 2: Check each option against the types

   { center: { x: 0, y: 0 }, radius: 5 } matches Circle exactly. { x: 0, y: 0, radius: 5 } has extra radius on a Point shape, invalid. { center: { x: 0 }, radius: 5 } has incomplete center. { x: '0', y: 0 } has x as string, invalid.

3. Final Answer:

   { center: { x: 0, y: 0 }, radius: 5 } -> Option A

4. Quick Check:

   Valid Shape = Circle or Point matching types [OK]

Hint: Match object shape exactly for union types [OK]

Common Mistakes:

• Mixing properties of union types incorrectly
• Using wrong property types inside objects
• Assuming partial objects are valid