TypeScript Type Issue

[unreal wyvern]

Hello, I'm having an issue which makes my brain hurt... I want that the type of "connection" is either "ITable" or "null" but for some reason it always throws me this error.

Code: https://img.walkaisa.dev/Kik9XWMSdC

Error at "this.connection" (Line: 7): Type 'null' is not assignable to type 'If<Ready, IConnection, null>'.
Error at "this.connection" (Line: 15): Type '{ id: string; timestamp: number; }' is not assignable to type 'If<Ready, IConnection, null>'.

[opal lark]

@unreal wyvern I'm not sure trying to express the readiness of the database through a generic type param is a good idea.

You're trying to assign to a generic conditional type there which isn't really a safe thing to do

[unreal wyvern]

But it will only be set to true in specific events where the database can only be ready.

What is even the issue in my code? I really have no plan.

@opal lark

[opal lark]

I just don't think generic classes really work the way you're trying to use them.

I would probably just have connection be IConnection | null.

Or - my preference - not construct the class itself until the connection is ready.

[unreal wyvern]

That's current work-around but I wanted to use the generic ones as it fits my use case the best.

[opal lark]

Generics aren't great for trying to express mutable, internal state of classes

That's genearlly just not something that's very ergonomic to try to reflect at the type level.

[unreal wyvern]

So you would stay at this one "IConnection | null"?

[opal lark]

Given your current setup, yeah.

Like I said, my preferred approach would be to make the connection then build the class.

[unreal wyvern]

Can you tell me at least what the issue is why it's showing that type error?

[unreal wyvern]

How would that look like?

[opal lark]

class Database {
    static connectToDatabase(): Promise<Database> {
         return makeConnection().then(connection => new Database(connection));
    }

    private connection: IConnection;
    constructor(connection: IConnection) {this.connection = connection;}   
}

Then things that need a database connection would either just have Database, which would be known to be connected, or Promise<Database> if it's possibly still connecting.

[opal lark]

It's just generally not safe to assign to conditional types.

At least not the kind that involve generics.

[unreal wyvern]

Yeah I understand that but I would still like to know what the issue is to learn it.

[opal lark]

I don't know what you mean.

That is the issue

[unreal wyvern]

What?

[opal lark]

Exactly.

[unreal wyvern]

I'm confused now.

[opal lark]

I just don't know what you're looking for. The issue is that generic conditional types are not things that are safe to assign to.

That's just now how they work.

[unreal wyvern]

Okay