Type alias for unions in Typescript - Mini Project: Build & Apply

Type alias for unions

📖 Scenario: You are building a simple program to categorize different types of pets by their names and types.

🎯 Goal: Create a type alias for a union of pet types, use it in a variable, and print the pet's type.

📋 What You'll Learn

Create a type alias called PetType that is a union of the strings 'dog', 'cat', and 'bird'.

Create a variable called myPet with the type PetType and assign it the value 'cat'.

Print the value of myPet.

💡 Why This Matters

🌍 Real World

Type aliases for unions help you restrict variables to specific allowed values, making your code safer and easier to understand.

💼 Career

Many jobs require writing clear and safe TypeScript code, especially when working with APIs or user input where only certain values are valid.

1

Create a type alias for pet types

Create a type alias called PetType that is a union of the strings 'dog', 'cat', and 'bird'.

Typescript

// Your code here

Hint

Use the type keyword followed by the alias name, then assign the union of string literals separated by |.

2

Create a variable with the type alias

Create a variable called myPet with the type PetType and assign it the value 'cat'.

Typescript

type PetType = 'dog' | 'cat' | 'bird';
// Your code here

Hint

Declare myPet with let, specify the type PetType, and assign the string 'cat'.

3

Use the variable in a function

Create a function called describePet that takes a parameter pet of type PetType and returns a string describing the pet type. Then call describePet with myPet and store the result in a variable called description.

Typescript

type PetType = 'dog' | 'cat' | 'bird';

let myPet: PetType = 'cat';
// Your code here

Hint

Define a function with parameter pet of type PetType that returns a string using a template literal.

4

Print the description

Print the value of the variable description using console.log.

Typescript

type PetType = 'dog' | 'cat' | 'bird';

let myPet: PetType = 'cat';

function describePet(pet: PetType): string {
    return `My pet is a ${pet}.`;
}

let description = describePet(myPet);
// Your code here

Hint

Use console.log(description) to print the description.

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