Determining the return type based on the parameter type

Here is an example of my function:

const safeIdCastToNumber = (id: string | null | undefined) => isNil(id) ? id : Number(id)

When calling safeIdCastToNumber, I can use an id parameter with a type union string | null | undefined, as well as one with a type union string | undefined for non-nullable ids.

The return type of this function is number | null | undefined.

I want the return type to exclude null if the id parameter does not include null.

For instance, if using an id parameter of type string | undefined, the return type should be number | undefined. However, if utilizing an id parameter of type string | null | undefined, the return type should be number | null | undefined.

Is it feasible to achieve this?

typescripttypesnarrowing

Answer №1

The answer will be shared here along with the use of overloaded functions.

function secureIdConversion(id: string | undefined): number | undefined
function secureIdConversion(id: string | null | undefined): number | null | undefined
function secureIdConversion(id: string | null | undefined) => isNil(id) ? id : Number(id)

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