Union type syntax and behavior in Typescript - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a union type in TypeScript?

A union type allows a variable to hold values of different types by combining multiple types with the | symbol. For example, string | number means the variable can be a string or a number.

beginner

How do you declare a variable that can be either a number or a boolean?

You use a union type like this: <code>let value: number | boolean;</code>. This means <code>value</code> can hold a number or a boolean.

intermediate

What happens if you try to use a union type variable without checking its type first?

TypeScript will show an error if you try to use properties or methods that are not common to all types in the union. You need to check the type first to safely use specific features.

intermediate

Explain how TypeScript narrows union types with an example.

TypeScript narrows union types by checking the type at runtime. For example:<br>

if (typeof value === 'string') {<br>  // Here, value is treated as string<br>  console.log(value.toUpperCase());<br>} else {<br>  // Here, value is treated as number<br>  console.log(value.toFixed(2));<br>}

beginner

Can union types include more than two types? Give an example.

Yes, union types can combine many types. For example: <code>let data: string | number | boolean;</code> means <code>data</code> can be a string, number, or boolean.

Which symbol is used to create a union type in TypeScript?

A ?

B &

C |

D :

The pipe symbol | is used to combine types into a union.

What will TypeScript do if you try to call a method that exists only on one type in a union without checking the type first?

A Automatically convert the type

B Show a type error

C Ignore the error

D Run the method anyway

TypeScript will show a type error because it cannot guarantee the method exists on all types in the union.

How can you safely use a union type variable that could be a string or a number?

A Use a type check like typeof

B Call string methods directly

C Assign it to any type

D Ignore the type system

Using typeof lets you check the type and safely use methods specific to that type.

Which of these is a valid union type declaration?

A let x: string | number;

B let x: string & number;

C let x: string, number;

D let x: string-number;

The correct syntax for union types uses the pipe | symbol.

Can union types include custom types like interfaces?

A Union types are only for arrays

B No, only primitive types are allowed

C Only classes can be used in union types

D Yes, union types can combine any types including interfaces

Union types can combine any types, including interfaces, classes, and primitives.

Describe how to declare and use a union type in TypeScript with an example.

Think about how a variable can hold a string or a number.

Explain how TypeScript helps you safely work with union types using type narrowing.

Consider how you check if a variable is a string before calling string methods.