Type Queries

Type queries allow us to obtain type information from values, which is an incredibly important capability — particularly when working with libraries that may not expose type information in a way that’s most useful for you

keyof

The keyof type query allows us to obtain type representing all property keys on a given interface

ts
type DatePropertyNames = keyof Date
            
type DatePropertyNames = keyof DateTry

Not all keys are strings, so we can separate out those keys that are symbols and those that are strings using the intersection operator (&).

If you remember your geometry, it may be useful to think of this as kind of like a dot product, in that when we use the intersection operator, we’re left only with the sub-part of the keyof Date that also is included by string or symbol, respectively.

ts
type DatePropertyNames = keyof Date
 
type DateStringPropertyNames = DatePropertyNames & string
               
type DateStringPropertyNames = "toString" | "toDateString" | "toTimeString" | "toLocaleString" | "toLocaleDateString" | "toLocaleTimeString" | "valueOf" | "getTime" | "getFullYear" | "getUTCFullYear" | ... 33 more ... | "getVarDate"
type DateSymbolPropertyNames = DatePropertyNames & symbol
               
type DateSymbolPropertyNames = typeof Symbol.toPrimitiveTry

Interesting! this Symbol.toPrimitive property is the only non-string. ¹

typeof

The typeof type query allows you to extract a type from a value. An example is shown below

ts
async function main() {
  const apiResponse = await Promise.all([
    fetch("https://example.com"),
    Promise.resolve("Titanium White"),
  ])
 
  type ApiResponseType = typeof apiResponse
             
type ApiResponseType = [Response, string]
}Try

A common use of typeof is to obtain a type representing the “static site” of a class (meaning: constructor, static properties, and other things not present on an instance of the class)

ts
class Fruit {
  constructor(
    public readonly name: string,
    public readonly mass: number,
    public readonly color: string
  ) {}
 
  static createBanana() {
    return new Fruit("banana", 108, "yellow")
  }
}
 
const MyFruit = Fruit
        
const MyFruit: typeof Fruit
const banana = Fruit.createBanana()
        
const banana: FruitTry

MyFruit, the class (constructor) is of type typeof Fruit, where instances are of type Fruit