Type narrowing

[meager blade]

How can I take an interface type and narrow it down to the specific class so I can access class-specific properties?

For example: how can I make it so that channel.type == 'guild' narrows the type from Channel to GuildChannel so I can access guild-specific properties?

[old crest]

p.e.t.e#0

Playground Link

Preview:```ts
// @strictPropertyInitialization: false
interface Channel {
type: 'guild' | 'dm';
cache: Message[];
send(msg: String): void;
}

class GuildChannel implements Channel {
type: 'guild';
cache: Message[];
send(msg: String): void { }

// guild-specific properties:

...```

You can choose specific lines to embed by selecting them before copying the link.

[strange flax]

the type of your channel property in the Message class should be a union of GuildChannel and DMChannel
you could make a helper type like this:```ts
type AnyChannel = GuildChannel | DMChannel

class Message {
channel: AnyChannel;
}

[meager blade]

ah, that works perfectly, thank you!

!resolved

[chilly sparrow]

fyi this is called a DU
https://www.typescriptlang.org/docs/handbook/2/narrowing.html#discriminated-unions

you could have Channel just have type: string in case you want to use extend it in the future

[meager blade]

I knew there was a name for it -- thanks so much! I had forgotten what it was called

[meager blade]

would typing if (message.channel.type == still give you a dropdown menu of possible values (guild, dm, etc) if you did it this way?

[chilly sparrow]

yes, because Channel would just be the base for extension, not in the actual structure (which would be AnyChannel)

you could also just omit type from Channel

[meager blade]

ah, I see

[meager blade]

but then implementing classes aren't required to have the type field, which sort of is the point of having an interface in the first place

like technically you could also omit send() from Channel couldn't you?

[chilly sparrow]

right, i misread your usage a bit. you'd want type there yeah

[chilly sparrow]

if not all channels have that then yes, for example stage channels or category channels

[meager blade]

(this is not really my use case -- I just used discord terms for the example)