Creating type aliases in Typescript - Quick Revision & Summary

Recall & Review

beginner

What is a type alias in TypeScript?

A type alias is a way to give a new name to a type. It helps make code easier to read and reuse by creating a simple name for complex types.

beginner

How do you create a type alias for a string in TypeScript?

You use the type keyword followed by the new name and the type. For example:

type Name = string;

intermediate

Can type aliases represent union types? Give an example.

Yes, type aliases can represent union types. For example:

type Status = "success" | "error" | "loading";

This means Status can be one of those three string values.

intermediate

What is the difference between a type alias and an interface in TypeScript?

Type aliases can name any type, including primitives, unions, and tuples. Interfaces describe object shapes and can be extended. Type aliases cannot be reopened to add new properties, but interfaces can.

beginner

Show how to create a type alias for an object with properties id (number) and name (string).

You write:

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

This creates a new type called User representing that object shape.

Which keyword is used to create a type alias in TypeScript?

A type

B alias

C interface

D class

The type keyword is used to create type aliases.

What does this type alias represent?

type ID = number | string;

A ID can only be a number

B ID is a function

C ID can be a number or a string

D ID is an object

The union number | string means ID can be either a number or a string.

Can a type alias be used to name a tuple type?

A Only with classes

B Yes

C Only with interfaces

D No

Type aliases can name tuple types, for example: type Point = [number, number];

Which of these can type aliases NOT do?

A Name object types

B Represent union types

C Name primitive types

D Extend other types by reopening

Type aliases cannot be reopened to add new properties or extend like interfaces can.

What is the correct syntax to create a type alias for an object with a string property 'title'?

A type Book = { title: string; };

B interface Book = { title: string; };

C class Book { title: string; }

D alias Book = { title: string; };

The correct syntax uses type followed by the name and the object shape.

Explain what a type alias is and how it helps in TypeScript programming.

Think about giving a nickname to a type.

Describe the difference between a type alias and an interface in TypeScript.

Focus on their capabilities and usage.

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