Implementing index signatures for derived types

Question

Implementing index signatures for derived types

I am facing an issue with a foundational type in my code:

export abstract class ElementBase extends HTMLElement {

    private state: StateBase;
    private propertyToWatch: string;

    update() {
        const externalValue = this.state[this.propertyToWatch]; // <- this doesn't compile
        // Element implicitly has an 'any' type because expression of type 'string' can't be used to index type 'StateBase'.
        this.updateUI(externalValue);
    }
...

The class StateBase has two methods and is inherited by various other classes that define properties like pageNumber: number;. I am unable to access the subclass properties using named indexing due to TypeScript v5.x deprecating the suppressImplicitAnyIndexErrors configuration switch. According to this guidance:

Fix: Add an index signature to the relevant type (or use a type assertion at the indexing location).

However, I am struggling to implement this for subtypes that are not known at runtime. If I add an index signature to StateBase, it disrupts the methods and properties of both the base type and its inheritors. Using a type assertion also seems impractical when dealing with subtypes. What could I be overlooking?

typescript

Answer 1

Answer №1

Shoutout to @Dimava for the idea in the comments, but here is how I tackled the problem. Typescript can be finicky about accessing properties using named indexes due to type safety concerns. However, you can still access those properties by utilizing the keyof syntax. Initially, I was unsure of how to apply this when dealing with a subtype without knowing the concrete type at runtime, but then I remembered generics exist in TS. The revised code snippet now looks like this:

export abstract class ElementBase<B extends StateBase> extends HTMLElement {

    private state: B; // using the generic type here
    private propertyToWatch: string;

    update() {
        const externalValue = this.state[this.propertyToWatch as keyof B]; // <- accessing the property with keyof
        this.updateUI(externalValue);
    }
...

In order to make this work, it was necessary to modify the class declaration to include the generic parameter. Additionally, I had to reference the subtype, generic B, instead of the base type StateBase. It's important that all subclasses of this new ElementBase declare the derivative of StateBase, which turned out to be straightforward.

I do wish they had kept the override config flag for this, but oh well. Just for those who are curious, the implementation was part of an observable pattern where components needed to update UI elements based on changes in specific state properties. While there are libraries available for this task, I preferred not adding them due to the limited scope of the project.

Answer 2

Shoutout to @Dimava for the idea in the comments, but here is how I tackled the problem. Typescript can be finicky about accessing properties using named indexes due to type safety concerns. However, you can still access those properties by utilizing the keyof syntax. Initially, I was unsure of how to apply this when dealing with a subtype without knowing the concrete type at runtime, but then I remembered generics exist in TS. The revised code snippet now looks like this:

export abstract class ElementBase<B extends StateBase> extends HTMLElement {

    private state: B; // using the generic type here
    private propertyToWatch: string;

    update() {
        const externalValue = this.state[this.propertyToWatch as keyof B]; // <- accessing the property with keyof
        this.updateUI(externalValue);
    }
...

In order to make this work, it was necessary to modify the class declaration to include the generic parameter. Additionally, I had to reference the subtype, generic B, instead of the base type StateBase. It's important that all subclasses of this new ElementBase declare the derivative of StateBase, which turned out to be straightforward.

I do wish they had kept the override config flag for this, but oh well. Just for those who are curious, the implementation was part of an observable pattern where components needed to update UI elements based on changes in specific state properties. While there are libraries available for this task, I preferred not adding them due to the limited scope of the project.

Implementing index signatures for derived types

Answer №1

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