Type alias for unions in Typescript - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a type alias in TypeScript?

A type alias gives a new name to a type. It helps make code easier to read and reuse by naming complex types.

beginner

How do you create a union type alias in TypeScript?

Use the type keyword followed by a name, then an equals sign, and list types separated by |. For example: type MyUnion = string | number;

beginner

What does the union type string | number mean?

It means a value can be either a string or a number. The value must match one of the types in the union.

intermediate

Why use a type alias for unions instead of writing the union everywhere?

It makes code cleaner and easier to update. If the union changes, you only update the alias once instead of many places.

intermediate

Can type aliases be used with complex unions like objects and primitives?

Yes! You can create aliases for unions of any types, including objects, primitives, or even other type aliases.

Which syntax correctly defines a type alias for a union of string and number?

A type MyType = string & number;

B alias MyType = string & number;

C alias MyType = string | number;

D type MyType = string | number;

The correct syntax uses 'type' keyword and '|' for union. So 'type MyType = string | number;' is correct.

What does the union type alias type Status = 'success' | 'error' | 'loading'; represent?

A A value that can be any string

B A value that must be exactly 'success', 'error', or 'loading'

C A value that can be a number or string

D A value that is always a boolean

The union lists specific string literals, so the value must be one of those exact strings.

Why might you use a type alias for a union type?

A To simplify code and make it easier to update

B To make the code harder to read

C To repeat the same union everywhere

D To avoid using unions

Type aliases help simplify code and make updates easier by naming complex types.

Which of these is NOT a valid union type alias?

A type Data = string & number;

B type Option = boolean | null;

C type Status = 'open' | 'closed';

D type Result = string | number;

'&' is intersection, not union. The question asks for union type alias.

Can a type alias for a union include other type aliases?

A Only if they are primitive types

B No, type aliases cannot be nested

C Yes, type aliases can be combined in unions

D Only if they are interfaces

Type aliases can be combined and reused inside unions freely.

Explain what a type alias for a union is and why it is useful in TypeScript.

Think about naming a group of types together.

Write a simple example of a type alias for a union of string literals representing colors.

Use single quotes for string literals.