Typescript: enforce object values without losing explicit keys

Question

In the following example, I'm able to extract explicit keys from the object, but i'm unable to enforce its values


const myCollection = {
    a: {test: 1},
    b: {test: 2},
    c: {text: 3} // no error
};

type MyCollectionKeys = keyof typeof myCollection;
// MyCollectionKeys is 'a' | 'b' | 'c'

When enforcing the values using the following snippet, you lose explicit keys:

type CollectionValue = {test: number};

const myCollection: ({[k: string]: CollectionValue}) = {
    a: {test: 1},
    b: {test: 2},
    c: {text: 3} // Type '{ text: number; }' is not assignable to type 'CollectionValue'.
};

type MyCollectionKeys = keyof typeof myCollection;
// MyCollectionKeys is string | number

A workaround I found is to proxy the object like so:

type CollectionValue = {test: number};

const _myCollection = {
    a: {test: 1},
    b: {test: 2},
    c: {text: 3}
};

type MyCollectionKeys = keyof typeof _myCollection;
// MyCollectionKeys is 'a' | 'b' | 'c'

const myCollection: ({[k in MyCollectionKeys]: CollectionValue}) = _myCollection;
// Type '{ a: { test: number; }; b: { test: number; }; c: { text: number; }; }' is not assignable to type '{ a: CollectionValue; b: CollectionValue; c: CollectionValue; }'.
//  Types of property 'c' are incompatible.
//    Property 'test' is missing in type '{ text: number; }' but required in type 'CollectionValue'

The drawbacks of this method are:

you must have a proxy object
if you violate the type, the error is thrown at the assignment instead of being thrown inside the original object at the offending key

Is there a more elegant solution for this?

brunnerh · Accepted Answer · 2020-10-21 15:18:18Z

3

Unfortunately generics cannot be partially inferred from use, so in this case i would probably just enumerate the valid keys:

const myCollection: Record<'a' | 'b' | 'c', { test: number }> = {
    a: {test: 1},
    b: {test: 2},
    c: {text: 3} // error
};

If you do not mind introducing a pointless function:

const check = <T extends string>(collection: Record<T, { test: number }>) => collection;

const myCollection = check({
    a: {test: 1},
    b: {test: 2},
    c: {text: 3} // error
});

answered Oct 21, 2020 at 15:18

brunnerh

188k30 gold badges362 silver badges434 bronze badges

Sign up to request clarification or add additional context in comments.

2 Comments

Ștefan-Gabriel Muscalu Over a year ago

I'm trying to avoid enumerating the keys myself as it is tedious. I'll give the function a try, as it will only run once at init and it's mostly a noop

Ștefan-Gabriel Muscalu Over a year ago

the function approach looks nice, and you can even make it generic: export function assertObjectRecordIntegrity<T>() { return <Keys extends string>(object: Record<Keys, T>) => object; }

Collectives™ on Stack Overflow

Typescript: enforce object values without losing explicit keys

1 Answer 1

2 Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

2 Comments

Your Answer

Sign up or log in

Post as a guest

Related