Get Object Type from Array of Keys

Question

I have a set of storage properties:

interface StoredProps {
  location: boolean;
  notification: boolean;
}

I also get the strings representing the stored properties using keyof:

type StorableKeys = keyof StoredProps;

I have a function that can retrieve multiple stored properties at once:

const = getStoredProps<T extends StorableKeys[]>(keys: T) =>
  keys.map(key => storage.get(key)).reduce((props, value, idx) => {
    props[keys[idx]] = value;
    return props;
  }, {});

This works, however the return type is ultimately {}. I want the return type to match an object created by the keys.

I've tried using {} as { [M in keyof T]: T[M] }, but this doesn't work because T is an array type so this just ends up giving me an array type. I've also tried [M in T], but this is a syntax error.

Is there any way to derive a type from an array of property names?

jcalz · Accepted Answer · 2018-05-17 19:12:25Z

2

Maybe something like this?

const getStoredProps = <K extends StorableKeys>(keys: K[]) =>
  keys.map(key => storage.get(key)).reduce((props, value, idx) => {
    props[keys[idx]] = value;
    return props;
  }, {} as Pick<StoredProps, K>);
}

The idea is to make the generic type just a union of keys K instead of an array of them, which is more straightforward. And then keys is of type K[]. The real value is the return type, Pick<StoredProps, K>, which is defined like {[P in K]: StoredProps[P]}, and it works now because K is a key type not an array type.

You might have been able to get away with T extends StorableKeys[] and then Pick<StoredProps, T[number]>, but I don't think you really mean to say that T could be a subtype of Array.

Hope that helps. Good luck.

answered May 17, 2018 at 19:12

jcalz

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