My current challenge involves creating a TypeScript method to flatten and select elements from paginated outputs. To illustrate, consider the following example:
const paginatedOutput: { results: number[], page: number, meta: string[] }[] = [{ page: 1, results: [1, 2, 3], meta: ['a', 'b', 'c'] }, { page: 2, results: [3, 4, 5], meta: ['d', 'e', 'f'] }];
flattenAndPick(paginatedOutput, ['results', 'meta']);
// Output:
//
// {
// results: [1, 2, 3, 4, 5, 6],
// meta: ['a', 'b', 'c', 'd', 'e', 'f']
// }
In essence, the flattenAndPick method combines array-type elements within objects stored in an array.
To tackle this issue, I have developed generic types and methods:
// Generic type for extracting properties of a specific type from an object
type PickByType<Obj, Type> = {
[key in keyof Obj as Obj[key] extends Type | undefined ? key : never]: Obj[key];
};
This custom type is crucial for selecting array properties from underlying array objects that signify pagination results needing concatenation.
The implementation of the selection and concatenation method is shown below:
// Method to flatten arrays within nested objects and concatenate selected data
const flattenAndPick = <T extends object, K extends keyof PickByType<T, any[]>>(
array: T[],
properties: K[],
): Partial<PickByType<T, any[]>> => {
const pickedObject: Partial<PickByType<T, any[]>> = {};
properties.forEach((property) => {
pickedObject[property] = [] as T[K];
});
array.forEach((element) => {
properties.forEach((property) => {
pickedObject[property] = (pickedObject[property] as any[]).concat(element[property]) as T[K];
});
});
return pickedObject;
};
I'm currently struggling to eliminate the type assertions in the above method. The error message states
Type 'undefined[]' is not assignable to type 'T[K]'The functionality of PickByType has been validated with examples provided in the code snippet.