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.