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.

Practice

1.

What does a type alias for unions in TypeScript do?

easy

A. Defines a function that returns different types

B. Creates a new variable with multiple values

C. Gives a single name to multiple possible types

D. Declares a constant with union values

Solution

1. Step 1: Understand the purpose of type alias

   A type alias lets you give a name to a type or group of types for easier use.

2. Step 2: Understand union types

   A union type means a value can be one of several types, like string or number.

3. Final Answer:

   Gives a single name to multiple possible types -> Option C

4. Quick Check:

   Type alias for unions = single name for multiple types [OK]

Hint: Type alias names multiple types combined with | [OK]

Common Mistakes:

• Thinking it creates variables instead of naming types
• Confusing union with function return types
• Assuming it declares constants

2.

Which of the following is the correct syntax to create a type alias MyType that can be either string or number?

easy

A. type MyType = string | number;

B. alias MyType = string & number;

C. type MyType = string & number;

D. alias MyType = string | number;

Solution

1. Step 1: Recall correct keyword for type alias

   Use the keyword type to create a type alias, not alias.

2. Step 2: Use union operator for multiple types

   The union operator is |, not &. So string | number is correct.

3. Final Answer:

   type MyType = string | number; -> Option A

4. Quick Check:

   type + union (|) = correct syntax [OK]

Hint: Use 'type' keyword and '|' for union types [OK]

Common Mistakes:

• Using 'alias' instead of 'type'
• Using '&' instead of '|' for unions
• Mixing syntax from other languages

3.

What will be the output of the following TypeScript code?

type Status = "success" | "error";

function printStatus(s: Status) {
  console.log(s);
}

printStatus("success");
printStatus("error");
// printStatus("pending"); // Uncommenting this line causes error

medium

A. No output

B. success\nerror\npending

C. Error at runtime

D. success\nerror

Solution

1. Step 1: Understand the type alias Status

   Status can only be "success" or "error" strings.

2. Step 2: Check function calls

   Calling printStatus with "success" and "error" prints those values. "pending" is invalid and commented out.

3. Final Answer:

   success\nerror -> Option D

4. Quick Check:

   Valid union values print correctly [OK]

Hint: Only allowed union values print without error [OK]

Common Mistakes:

• Assuming invalid union values print without error
• Thinking it causes runtime error
• Ignoring commented invalid call

4.

Identify the error in this TypeScript code using a type alias for unions:

type ID = number | string;

let userId: ID = true;

medium

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

B. Missing semicolon after type alias

C. Type alias cannot use union types

D. Variable 'userId' must be declared with 'const'

Solution

1. Step 1: Check type alias definition

   ID allows only number or string values.

2. Step 2: Check assigned value type

   Assigning true is boolean, which is not allowed by the union.

3. Final Answer:

   Type 'boolean' is not assignable to type 'number | string' -> Option A

4. Quick Check:

   Assigning wrong type causes error [OK]

Hint: Assigned value must match union types exactly [OK]

Common Mistakes:

• Thinking union allows any type
• Ignoring type mismatch errors
• Confusing syntax errors with type errors

5.

Given the type alias type Response = { status: "ok"; data: string } | { status: "error"; error: string };, which of the following objects matches the Response type?

hard

A. { status: "ok", error: "File missing" }

B. { status: "ok", data: "File loaded" }

C. { status: "error", data: "File loaded" }

D. { status: "error", error: 404 }

Solution

1. Step 1: Analyze the first union type

   When status is "ok", object must have a string property named data.

2. Step 2: Analyze the second union type

   When status is "error", object must have a string property named error.

3. Step 3: Check each option

   { status: "ok", data: "File loaded" } matches first type correctly. Options B and C mix properties incorrectly. { status: "error", error: 404 } has error property as number, not string.

4. Final Answer:

   { status: "ok", data: "File loaded" } -> Option B

5. Quick Check:

   Match status with correct property name and type [OK]

Hint: Match status value with correct property name and type [OK]

Common Mistakes:

• Mixing properties from different union types
• Using wrong property types
• Ignoring the status-property relationship