keyof typeof Object

Introduction

I'm going to talk about keyof typeof in typescript and how it could be helpful. I will start explaining typeof and key separate and in which case you might want to use it.

Literal types

Literal types in TypeScript are more specific types of string, number or boolean. Let's elaborate with an example, "Hey" is a string, but a string is not "Hey". "Hey" is a more specific type of type string, so it is a literal type.

A literal type can be declared as following:

type Saludo = "Hey"

This means that the object of type Saludo can have only a string value "Hey" and no other string value or any other value of any other type as shown in the following code:

let saludo: Saludo
saludo = "Hey" //Ok
saludo = "hi" // Error: Type "hi" is not assignable to type "Hey"

Literal types are not useful on their own, however when combined with union types, type aliases and type guards they become powerful.

Following is an example of union of literal types:

type Saludo = "Hey" | "Hi" | "Welcome"

Now the object of type Saludo can have the value either "Hey", "Hi" or "Welcome".

let saludo: Saludo
saludo = "Hey" //Ok
saludo = "Hi" //Ok
saludo = "Welcome" //Ok
saludo = "Good" //Error: type "Good" is not assignable to type Saludo

keyof

keyof of some type T gives you a new type that is a union of literal types and these literal types are the names of the properties of T. The resulting type is a subtype of string.

For example, consider the following interface:

interface Person {
 name: string
 age: number
 location: string
}

Using the keyof operator on the type Person will give you a new type as shown in the following code:

type SomeNewType = keyof Person

This SomeNewType is a union of literal types ("name" | "age" | "location") that is made from the properties of type Person.

Now you can create objects of type SomeNewType:

let newTypeObject: SomeNewType
newTypeObject = "name" //ok
newTypeObject = "age" //ok
newTypeObject = "location" //ok
newTypeObject = "other" //Error..

keyof typeof together on an object

The typeof operator gives you the type of an object. In the above example of Person interface, we already knew the type, so we just had to use the keyof operator on type Person.

But what to do when we do not know the type of an object or we just have a value and not a type of that value like the following?

const bmw = {name: "bmw", power: "100ph"}

This is where we use keyof typeof together.

The typeof bmw gives you the type: {name: string, power: string}

Ant then keyof operator gives you the literal type union as shown in the following code:

type CarLiteralType = keyof typeof bmw;

let carPropertyLiteral: CarLiteralType
carPropertyLiteral = "name" //Ok
carPropertyLiteral = "power" //Ok
carPropertyLiteral = "other" //Error

Basically what it means is that if the value is inside of the object we will be able to call the property since the keyof typeof give us that advantage.

Conclusion

I found really interesting this use of keyof type object and I recommend to use it. I will be posting more things about typescript and how to type react components.

Cheers, Lautaro.