6 Ways to Use Extract in TypeScript

Just like my previous article on Exclude, I’ve put together a bunch of different use cases for one of TypeScript's most useful helpers - Extract.

#

Extract a member of a discriminated union by discriminator

Given a discriminated union, like Event below, you often need to be able to access a single member of the union.

This can feel like an obscure problem, until you realize that Extract is the perfect tool for the job.

Here, we're extracting the ClickEvent type from the Event:

type Event =
  | {
      type: "click";
      x: number;
      y: number;
    }
  | {
      type: "focus";
    }
  | {
      type: "blur";
    };

type ClickEvent = Extract<Event, { type: "click" }>;
         
type ClickEvent = {
    type: "click";
    x: number;
    y: number;
}

We pass Event as the first argument to Extract, and then we pass the shape of the member we want to extract as the second argument.

Crucially, we don't need to pass the entire member to Extract, just the shape we want to match. Beautiful.

#

Extract a member of a discriminated union by shape

Sometimes, we want to extract a specific member of a discriminated union based on its shape, rather than its discriminator.

Let's go back to our Event type we declared earlier:

type Event =
  | {
      type: "mousedown";
      x: number;
      y: number;
    }
  | {
      type: "mouseup";
      x: number;
      y: number;
    }
  | {
      type: "blur";
    };

Instead of extracting a member based on its discriminator, let's say we want to extract a member that has an x property of type number.

We can provide { x: number } as the shape to match:

type ClickEvent = Extract<Event, { x: number }>;
         
type ClickEvent = {
    type: "mousedown";
    x: number;
    y: number;
} | {
    type: "mouseup";
    x: number;
    y: number;
}

The result is the ClickEvent type, which is the member of the Event union that has an x property of type number. In this case, since both the mousedown and mouseup members have an x property, the ClickEvent type will be a union of these two members.

#

Extract all strings/booleans/numbers from a union

Let's consider a union type that includes a mix of strings, booleans, and numbers:

type PossibleValues =
  | "admin"
  | "user"
  | "guest"
  | true
  | false
  | 1
  | 2
  | 3;

To extract all the string members from this union, we can use Extract<PossibleValues, string>:

type Strings = Extract<PossibleValues, string>;
       
type Strings = "admin" | "user" | "guest"

The result will be the Strings type, which is a union of all the string members ("admin", "user", and "guest") in the PossibleValues union.

#

Extract all functions from a union

Let's say we have a union type that includes a mix of strings, numbers, booleans, and functions:

type PossibleValues =
  | string
  | number
  | boolean
  | (() => void);

To extract all the function members from this union, we can use Extract<PossibleValues, (...args: any[]) => any>:

type Functions = Extract<
        
type Functions = () => void
  PossibleValues,
  (...args: any[]) => any
>;

(...args: any[]) => any effectively matches any function type - so we can use it to extract all the function members from the union.

In this case, the Functions type will only contain the () => void function type.

#

Exclude null and undefined from a union

There are times when we want to exclude certain types from a union. We may have a union type that includes null and undefined, but we want to create a new type that excludes those values.

type PossibleValues =
  | string
  | number
  | boolean
  | null
  | undefined;

To exclude null and undefined from PossibleValues, we can use Extract<PossibleValues, {}>.

type NotNull = Extract<PossibleValues, {}>;
       
type NotNull = string | number | boolean

We can use {} as the second argument to Extract to match all types EXCEPT for null and undefined. You can learn more in this article on the empty object type.

#

Find common members between two unions

In the following example, we have two union types: EnglishSpeakingCountries and CountriesInWesternHemisphere:

type EnglishSpeakingCountries = "UK" | "USA" | "Canada";
type CountriesInWesternHemisphere =
  | "USA"
  | "Canada"
  | "Mexico";

To find the common countries between these two unions, we can use the Extract utility type like this:

type CommonCountries = Extract<
           
type CommonCountries = "USA" | "Canada"
  EnglishSpeakingCountries,
  CountriesInWesternHemisphere
>;

The resulting CommonCountries type will contain only the members that are present in both unions. In this case, the common country is "USA" as it exists in both unions.