CODEWITHSUNDEEP
typescript
ffxsam
ffxsam

Reputation: 27793

How to remove fields from a TypeScript interface via extension

Let's say there's an interface that I don't have control over:

interface Original {
  name: string;
  size: number;
  link: SomeType;
}

I want to extend this interface, but actually remove link so I end up with a new interface that is effectively:

interface OriginalLite {
  name: string;
  size: number;
}

How can I do this?

Upvotes: 90

Views: 61692

Answers (3)

jcalz
jcalz

Reputation: 330571

You can use mapped and conditional types to achieve this. Usually we call what you're trying to do Omit:

type Omit<T, K extends keyof T> = Pick<T, Exclude<keyof T, K>>

(for TS3.5+, the Omit utility type is included in the standard library, so you don't have to define it yourself anymore)

So Omit<T, K> is basically the same as T but with none of the properties in K. You can do the following to make a new interface called OriginalLite which is missing only link:

interface OriginalLite extends Omit<Original, 'link'> {}

Note that you are not extending Original. That's because to "extend" in TypeScript means to make a type more specific (a.k.a. "narrowing"), but in your case what you are doing is making it less specific (a.k.a. "widening"). So that's why extends Original does not appear in that definition. I assume that's okay with you.

Let's test it:

declare const originalLite: OriginalLite;
originalLite.name; // string
originalLite.size; // number
originalLite.link; // error, property 'link' does not exist on type 'OriginalLite'

Looks good.

Upvotes: 189

Covik
Covik

Reputation: 862

Just to add: if you don't have a list of keys you want to remove but only interface you can use keyof to get a list of keys and omit them.

Example:

interface RemoveThoseKeys {
  removeMe1: unknown
  removeMe2: unknown
}

interface AllKeys {
  keep1: unknown
  keep2: unknown
  removeMe1: unknown
  removeMe2: unknown
}

type KeysAfterRemoval = Omit<AllKeys, keyof RemoveThoseKeys>

If you instantiate an object with type KeysAfterRemoval and hit auto-completion in your IDE you should get only keep1 and keep2.

IDE autocompletion

Upvotes: 11

Shah
Shah

Reputation: 2894

Sample of omitting multiple unwanted properties from a type, and adding new custom properties.

type UnwantedKeys = "key1" | "key2"

interface MyNewType extends Omit<Original, <UnwantedKeys>> {
  newProp1: string
  newProp2: number
}

Upvotes: 13

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