Is there a way to call array prototype methods on union types?

Question

Consider following code as example:

    class Something {

      public getItems(): string | string[] {

        return items;
      }

    } 

Is it Possible to access to the array prototype method on calling getItems() ?

Something like this ?

let something = new Something();

const items = something.getItems();

items.filter(i => i.length > 2); // here for example 

Answer 1

The only issue why you cant is the type getItems returns and it is string | string[]. The string part is a problem, as string has no Array prototype. In order to use the value returned by getItems we need to narrow the type by type guard.

const items = something.getItems(); // items is string | string[]

if (typeof items !== 'string') {
  // now type is narrowed to string[]
  items.filter(i => i.length > 2); // fully type safe
} else {
  // here you have a string
}

We can also do that more consist by narrowing the structure to the array of strings also for string.

const items = something.getItems(); // items is string | string[]
// in line below I am bringing both possible values into array structure
const finalItems = typeof items === 'string' ? [items] : items; 
finalItems.filter(i => i.length > 2); // fully type safe

The important in above is [items] where I put single string value into array, thanks that finalItems has only one type - string[]