Filtering in TypeScript

#functional #typescript #codenewbie #webdev

Say that you have this type:

type List = (Subject | null | undefined)[]

Perhaps it is the result of querying for a list of identifications, where not every entry is guaranteed to have a result.

Nothing strange so far. However, let's say you now want to remove null and undefined elements, in order to do a further operation on the leftover Subject elements.

interface Subject {
    id: string | number
    name: string
}

type List = (Subject | null | undefined)[]

const joe: Subject = { id: 0, name: 'joe' }
const jane: Subject = { id: 10, name: 'jane' }
const list: List = [joe, null, undefined, jane]

const names = list.filter(entry => !!entry).map(entry => entry.name)

console.log(names)

Unfortunately this does not work:

TSError: ⨯ Unable to compile TypeScript:
index.ts:12:58 - error TS2533: Object is possibly 'null' or 'undefined'.

We need to tell TypeScript that the filter operation ensures only Subject types are left.

function exists<T>(value: T | null | undefined): value is T {
    return value === (value ?? !value)
}

The exists function, for example, makes sure to assert that value is of type T.

In production code, try to avoid generic naming such as T

To filter and have Subject types as leftover:

interface Subject {
    id: string | number
    name: string
}

type List = (Subject | null | undefined)[]

const joe: Subject = { id: 0, name: 'joe' }
const jane: Subject = { id: 10, name: 'jane' }
const list: List = [joe, null, undefined, jane]

function exists<T>(value: T | null | undefined): value is T {
    return value === (value ?? !value)
}

const names = list.filter(exists).map(entry => entry.name)

console.log(names) // [ "joe", "jane" ]