What is the reason behind not being able to perform a null check on an array entry in Typescript

I've got an array filled with objects that may be either null or a Gamepad:

let devices: (Gamepad | null)[] = navigator.getGamepads()

If the first item in the array happens to be a valid Gamepad, I need to perform a specific action:

let device: Gamepad | null = devices[0]
if (device) {
     handleDevice(device)
}

The current solution involves creating an additional temporary variable for null-checking. Is there any way to directly check the first entry of the array for null?

if (devices[0]) {
     handleDevice(devices[0])
}

Argument of type 'Gamepad | null' is not assignable to parameter of type 'Gamepad'. Type 'null' is not assignable to type 'Gamepad'

The function I'm calling accepts a gamepad as its argument:

function handleDevice(g:Gamepad) {
    console.log(g)
}

Answer №1

This particular issue involves a bug in the compiler related to index signatures for types like arrays not being properly type guarded when accessing index properties using square brackets. The timeline for fixing this bug is uncertain, with the current milestone indicating "TypeScript 3.5", though it seems unlikely to be achieved given the upcoming release of that version shortly after the stated date (as of May 11, 2019). As you mentioned, a workaround is to assign the value to a new variable and perform the necessary checks. Best of luck resolving this issue!

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