Transform array of objects to a map indexed by object key

Question

I'm trying to convert an array of object to a map, indexed by an attribue value of the Object in typescript 4.1.5

Furthermore I want only the attributes of a certain type (here string)

A very similar question have been asked in javascript here : Convert object array to hash map, indexed by an attribute value of the Object

type Foo = {
        a : string
        b : number
}

const foos: Foo[] = []
// should only propose to index by 'a'
const foos_indexed_by_a = indexArrayByKey(foos, 'a')

function indexArrayByKey<T> (array: T[], key: Extract<keyof T, string>): Map<string, T> {
        return array.reduce((map, obj) => map.set(obj[key], obj), new Map<string, T>())
}

Playground Link

For now I can't access property key of obj (compilation failure)

Thanks for your help :)

Titian Cernicova-Dragomir · Accepted Answer · 2022-01-20 09:16:26Z

2

To filter the keys based on which have a string value you can't use Extarct that filters the keys which are strings (not number or symbol). You can use KeyOfType described here

type KeyOfType<T, V> = keyof {
    [P in keyof T as T[P] extends V? P: never]: any
}
function indexArrayByKey<T, K extends KeyOfType<T, string>> (array: T[], key: K): Map<T[K], T> {
        return array.reduce((map, obj) => map.set(obj[key], obj), new Map<T[K], T>())
}

Playground Link

We use T[K] instead of string because T[K] could also be a subtype of string so TS would complain at string, but for cases this will work well.

answered Jan 20, 2022 at 9:16

Titian Cernicova-Dragomir

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5 Comments

T.J. Crowder Over a year ago

I think you were right my answer was missing the point. The question clearly says "indexed by an attribue value of the Object". sigh :-)

Nainpo Over a year ago

Thanks your answer works really well in typescript v4.5.4 but not in the one I'm using v4.1.5. I should have specified it sorry. Do you have a solution for it ?

Titian Cernicova-Dragomir Over a year ago

@Nainpo you can just swap out the definition of KeyOfType with the older one here stackoverflow.com/questions/51419176/… The rest should work.

Titian Cernicova-Dragomir Over a year ago

Playground link for older compatible version: typescriptlang.org/play?ts=4.1.5#code/…

Nainpo Over a year ago

Thanks for the new definition it's ok ! :)

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