Ways to inform TypeScript of the potential return type when a generic's parameter can be either a string or a number

Let's take a look at a function with the following signature:

function removeNumbersOrStringsElementsFromArray(
    targetArray: Array<number | string>,
    targetElementOrMultipleOfThem: number | string | Array<number | string>
): {
  updatedArray: Array<number | string>;
  removedElementsIndexes: Array<number>;
} {
  // implementation
}

Even if all the elements in targetArray are either all strings or all numbers,

const result: Array<string> = removeNumbersOrStringsElementsFromArray([ 1, 2, 3, 4 ], [ 1, 2 ]).updatedArray;

this will result in an error:

“Type (string|number) is not assignable to type string[]”

One solution could be to change the signature from Array<number | string> to 

Array<number> | Array<string>
, but that would require changing the implementation of 
removeNumbersOrStringsElementsFromArray
. However, from a JavaScript perspective, it could work. Are there any other solutions?

arraystypescript

Answer №1

function extractElementsFromArray< T extends string | number>(
    inputArray: Array<T>,
    elementsToRemove: T | Array<T>
): {
    modifiedArray: Array<T>;
    removedIndexes: Array<number>;
} {

}

const newArray: Array<number> = extractElementsFromArray([5, 6, 7, 8], [6, 7]).modifiedArray;

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