Typescript's keyof with array of objects

Question

I'm very new to typescript and having a hard time finding some extensive documentation on keyof. If you have

export interface someClassProps<T extends object> {

    /*
        Lets assume T look is an object that looks something like:

        T = {"name": "",
             "val": NaN,
             "active": false}
    */

    A: Array<T>;            // an array of objects
    B: keyof T;             // What type is this? Is just a type of object or is it the keys of T?
    C: keyof Array<T>[0];   // Is this the keys of T?
    D: keyof Array<T>       // Is this type object?
}

What type do you get for B or C? I'm hoping to get a type from keyof that is the would be name | val | active. Am I looking for an approach that looks like B, C, or something fully different?

Alternatively is there an easy way to print the types from keyof? That would allow me to just figure this out.

Explosion Pills · Accepted Answer · 2018-05-21 17:04:50Z

2

keyof gives you the union type of the keys of the given type. So for example, if you had:

export SomeClass implements<{ name: string, val: number, active: boolean}>

...then B would be "name" | "val" | "active". In this case, C would also be "name" | "val" | "active" since Array<T>[0] is 0th element of an array with elements of T which is just T.

Note that with the way TypeScript interfaces work, implements just means "check this implementation is assignable to the interface" and doesn't actually set the class types by default. This means you will still have to do B: "name" | "val" | "active" if you wanted. You could also just do B: "name" since that type is assignable to the union type of all the keys.

answered May 21, 2018 at 17:04

Explosion Pills

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4 Comments

Grant Williams Over a year ago

So this function is being used in a way that i don't know what the keys of the object are. Is B always guaranteed to be the same as C if Array<T> is an array of objects? Or is it keyof Array<T> just going to be of type object?

Explosion Pills Over a year ago

keyof Array<T> is different than keyof Array<T>[0] since the former would be the union type of all keys of Array including array methods. Otherwise it will be the T object.

Grant Williams Over a year ago

Ah i see. So i've added a new typeD to my example. In my example B and C are the same type, but D is not?

Explosion Pills Over a year ago

Type D would essentially be the union of "includes" | "length" | "map", etc. i.e. you probably don't want to use that

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