Type alias for unions in Typescript - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of union type alias usage

What is the output of this TypeScript code when compiled and run with Node.js?

Typescript

type Status = "success" | "error";

function respond(status: Status): string {
  if (status === "success") {
    return "Operation succeeded";
  } else {
    return "Operation failed";
  }
}

console.log(respond("success"));

A "Operation succeeded"

B Compilation error

C TypeError at runtime

D "Operation failed"

💡 Hint

Check what the function returns when the input is "success".

The type alias Status allows only "success" or "error". The function returns "Operation succeeded" when status is "success".

❓ Predict Output

intermediate

Value of variable with union type alias

What is the value of variable 'result' after running this code?

Typescript

type ID = number | string;

let userId: ID = 123;
userId = "abc";
const result = typeof userId;

A "number"

B "string"

C "undefined"

D "object"

💡 Hint

Look at the last assignment to userId and what typeof returns.

userId is assigned the string "abc" last, so typeof userId is "string".

🔧 Debug

advanced

Identify the error in union type alias usage

What error does this TypeScript code produce?

Typescript

type Response = { code: number } | { message: string };

function handleResponse(res: Response) {
  console.log(res.code);
}

handleResponse({ message: "Error" });

A Type 'string' is not assignable to type 'number'.

B No error, prints undefined

C Property 'code' does not exist on type '{ message: string; }'.

D Function handleResponse is missing return type

💡 Hint

Check if all union members have the property 'code'.

The union type Response can be either {code: number} or {message: string}. The code tries to access res.code without checking which type it is, causing a TypeScript error.

📝 Syntax

advanced

Correct syntax for union type alias

Which option correctly defines a type alias 'Shape' that can be either 'circle' or 'square'?

A type Shape = "circle" | "square";

B type Shape = "circle", "square";

C type Shape = "circle" & "square";

D type Shape = ("circle" | "square")[];

💡 Hint

Union types use the pipe symbol | between types.

Option A correctly uses the union operator | to define Shape as either "circle" or "square".

🚀 Application

expert

Determine the number of possible values for a union type alias

Given this type alias, how many distinct values can a variable of type 'Level' hold?

Typescript

type Level = "low" | "medium" | "high" | 1 | 2 | 3;

A 1

B 3

C 9

D 6

💡 Hint

Count all unique literals in the union.

The union has 3 string literals and 3 number literals, total 6 distinct values.