Trying to determine the argument type, but unsure of the next steps
function convertArrayToObject(keys) {
return keys.reduce((prev, key) => ({ ...prev, [key]: key}), {})
}
const output = convertArrayToObject(['aa', 'bb']);
// expected output type : `{ aa: string, bb: string }`
To prevent the compiler from widening the type of the passed array to string[], you will need to utilize const type parameter.
It's important to note that const type parameters are only available in Typescript version 5.0. In versions prior to that, a possible solution, although a bit cumbersome, is to use const assertion, as demonstrated in @Behemoth's answer.
function convertArrayToObject<const T extends readonly string[]>(keys: T) {}
Once that is in place, the type for the reduce function needs to be defined. It should be an object where the keys are the elements of the array and the values are regular strings.
To determine the type of elements, indexed access is required:
// 'aa' | 'bb'
type Case1 = ['aa', 'bb'][number]
Adapting this concept to fit our required object:
// type Case2 = {
// aa: string;
// bb: string;
// }
type Case2 = {[K in ['aa', 'bb'][number]]: string}
Looking good so far!
We then need to assert the initial value of the reduce function to this type:
function convertArrayToObject<const T extends readonly string[]>(keys: T) {
return keys.reduce((prev, key) => ({ ...prev, [key]: key }), {} as {[K in T[number]]: string});
}
For testing purposes:
// const result: {
// aa: string;
// bb: string;
// }
const result = convertArrayToObject(['aa', 'bb']);
Try it out in the playground
To enhance the functionality of the convertArrayToObject() method, you can modify it to accept a generic parameter that will help infer the return type. However, to ensure compatibility, you will need to employ Type Assertion for the initial value in the .reduce() method to align with the return type. It is essential to specify your argument array as const so that the values can be accurately inferred as more than just string data types.
function convertArrayToObject<T extends readonly string[]>(
keys: T
): {[K in T[number]]: string} {
return keys.reduce(
(prev, key) => ({...prev, [key]: key}),
{} as {[K in T[number]]: string}
);
}
const result = convertArrayToObject(["aa", "bb"] as const);
// ^? const result: { aa: string; bb: string; }
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