Indexed access types in Typescript - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is an indexed access type in TypeScript?

An indexed access type lets you look up the type of a property in another type using square brackets, like TypeName[Key]. It extracts the type of that property.

beginner

How do you use indexed access types to get the type of a property 'name' from an interface Person?

You write Person['name']. This gives you the type of the 'name' property defined in Person.

intermediate

Can indexed access types use union types as keys? Give an example.

Yes. For example, if key is 'name' | 'age', then Person[key] is the union of the types of name and age properties.

intermediate

What happens if you use an indexed access type with a key that does not exist on the type?

TypeScript will give an error because the key is not assignable to the keys of the type. Indexed access types require valid keys.

intermediate

Explain how indexed access types help with type safety in functions.

They let you write functions that accept keys of an object and return the exact type of the property at that key, ensuring you don't mix types accidentally.

What does Person['age'] represent in TypeScript?

A The value of the 'age' property in Person

B An error because 'age' is a string

C A new object with only the 'age' property

D The type of the 'age' property in Person

Person['age'] extracts the type of the 'age' property from the Person type.

Can you use a union of keys in indexed access types like Person['name' | 'age']?

A Yes, it returns a union of the property types

B No, it causes a syntax error

C Yes, but only if the keys are numbers

D No, it returns the first key's type only

Using a union of keys returns a union of the types of those properties.

What error occurs if you use an invalid key in an indexed access type?

A No error, it returns <code>any</code>

B Type error: key not assignable to keys of the type

C Runtime error

D Syntax error

TypeScript enforces keys must exist on the type, otherwise it reports a type error.

Which of these is a valid use of indexed access types?

A type AgeType = Person['age'];

B let age: Person['age'] = 'twenty';

C Person['height'] = 180;

D type Name = Person.name;

Option A correctly extracts the type of 'age' property. Option B may be invalid if 'age' is not a string. Option C tries to assign a value to a type. Option D uses dot notation incorrectly for types.

How do indexed access types improve function definitions?

A By allowing functions to ignore types

B By making functions run faster

C By allowing functions to return the exact property type based on a key parameter

D By converting all types to strings

Indexed access types help functions be more precise about the types they return based on input keys.

Describe what indexed access types are and how they are used in TypeScript.

Think about how you get a property type from an object type.

Explain how indexed access types can help make your TypeScript code safer when working with object properties.

Consider how knowing exact property types prevents errors.