Get type inference from keys in a dynamic array in TypeScript

Question

I am looking to get type inference based on the value of keys in an array.

For context, I'm working on a class where you can pass in a list of options and a default value:

interface Option {
    value: string;
}

interface SelectArgs {
    options: Option[];
    defaultValue: Option['value'];
}

class SelectConfig {
    options: Option[];
    defaultValue: Option['value'] // can I get the value options with autocompletion here?

    constructor(args: SelectArgs ) {
        Object.assign(this, args);
    }

}

I would be using this in the context of a class, so for example, if I instantiate a new class:

const select = new SelectOptions({
    options: [{value: 'Hello'}, {value: 'World'}]
    defaultValue: "Hello" // I want to give me type inference of either "Hello" | "World"

})

What is the best way to type defaultValue in this case so it is always one of the passed in values of options?

k.tten · Accepted Answer · 2022-09-22 19:12:57Z

3

Yes, it'll look something like this, using a generic parameter for the class:

type Narrow<T> =
    | (T extends infer U ? U : never)
    | Extract<T, number | string | boolean | bigint | symbol | null | undefined | []>
    | ([T] extends [[]] ? [] : { [K in keyof T]: Narrow<T[K]> });

class SelectConfig<Options extends Option[]> {
    options!: Options;
    defaultValue!: Options[number]['value'] 

    constructor(args: {
        options: Narrow<Options>;
        defaultValue: Options[number]["value"]
    }) {
        Object.assign(this, args);
    }
}

I opted to use this Narrow utility type so you don't need to use as const when creating the class. We'll narrow the type of the given options, then set the type of the default value to any of the values given in the options.

Playground

edited Sep 22, 2022 at 19:12

answered Sep 22, 2022 at 19:07

k.tten

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6 Comments

Alex Wayne Over a year ago

The instance members should be using the generic Options here, not Option.

k.tten Over a year ago

Nice catch, for some reason I was concerned that later on when you used the members it would give you type errors because the members were too specific (adding options, etc), but it looks like it doesn't.

Viktor Luft Over a year ago

Nice. Didn't know yet about this Narrow utility. Where did you find it?

k.tten Over a year ago

@ViktorLuft I found it in the TypeScript community Discord server. It's very commonly used, so much so that their Discord bot has a command to bring this type up.

Alex Wayne Over a year ago

Also, as const is probably fine, and arguably a lot simpler. Regardless, nice answer, up voted!

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