How to model an possible empty array with Typescript

Question

I have the current scenario.

type A = { b: string, c: number }

an object that I'll consume from an API , that will give me either

A[] || []

Currently, if I try to use it,

const apiData: A[] || []
const b = apiData[0].a // this I wanted it to error, but it doesn't.

How do I model in a way where this will error ?

const b = apiData[0].a definitely does error. Also you have a typo, its A[] | [], not A[] || [] — Roberto Zvjerković
– Roberto Zvjerković, Commented Jun 9, 2021 at 12:31

captain-yossarian from Ukraine · Accepted Answer · 2021-07-12 06:10:44Z

1

I believe the simplest solution here is Partial:

type A = { b: string, c: number }

type Obj = Partial<A[]>
declare var apiData: Obj;

const check = apiData[0] // A | undefined

OR you can use typeguard

type A = { b: string, c: number }

type Obj = A[]
declare var apiData: Obj;

const isEmpty = <Elem,>(arg: Elem[]): arg is never[] => arg.length === 0

if (isEmpty(apiData)) {
  const x = apiData[0] // never
} else {
  const x = apiData[0] // A
}

Next type

type Obj = A[] | never[]

does not work as you might expect because never[] extends A[]

type IsAssignable = never[] extends A[] ? true : false // true

Also, you can use noUncheckedIndexedAccess flag

edited Jul 12, 2021 at 6:10

answered Jun 9, 2021 at 12:23

captain-yossarian from Ukraine

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