Is there a way to create a Typescript function that can automatically return either a scalar or array value without requiring the caller to manually cast the output

Question

Is there a way to create a Typescript function that can automatically return either a scalar or array value without requiring the caller to manually cast the output

Challenge

Looking for a solution to the following problem:

save<T>(x: T | T[]) {
 if (x instanceof Array) {
     // save array to database
 } else {
     // save entity to database
 }
 return x
}

// client code
let obj: SomeType = { // values here }
let result = save(obj) // confusion arises during compilation...

The issue lies in TypeScript's uncertainty about whether result is expected to be singular or an array.

Inquiry

How can I modify the function to specify that:

if given a singular parameter, TypeScript recognizes the output as singular
if given an array parameter, TypeScript identifies the output as an array

typescript

Answer 1

Answer №1

Modify the definition by removing T[]. This is because T[] is a subset of T.

function update<T>(item: T): T {
    if (item instanceof Array) {
        // update array in database
    } else {
        // update entity in database
    }

    return item;
}

// example usage
let dataA: SomeDataType = { /* values here */ };
let updatedDataA = update(dataA); // updatedDataA has type SomeDataType

let dataB: SomeDataType = { /* values here */ };
let updatedDataB = update([dataA, dataB]); // updatedDataB has type SomeDataType[]

Answer 2

Modify the definition by removing T[]. This is because T[] is a subset of T.

function update<T>(item: T): T {
    if (item instanceof Array) {
        // update array in database
    } else {
        // update entity in database
    }

    return item;
}

// example usage
let dataA: SomeDataType = { /* values here */ };
let updatedDataA = update(dataA); // updatedDataA has type SomeDataType

let dataB: SomeDataType = { /* values here */ };
let updatedDataB = update([dataA, dataB]); // updatedDataB has type SomeDataType[]

Is there a way to create a Typescript function that can automatically return either a scalar or array value without requiring the caller to manually cast the output

Challenge

Inquiry

Answer №1

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