I’m trying to figure out why TypeScript allows array access in the first case below but not the second. I have noUncheckedIndexedAccess enabled, but I’m checking that the accessed objects are not undefined before accessing them.

This doesn’t error:

const arr: number[][] = [[], [], []];
if (arr[0] != undefined) {  
    arr[0][0] = 42;  
}

But this does:

function twoDimArrayAccess(arr: number[][], x: number, y: number) {
    if (arr[x] != undefined) {
        arr[x][y] = 1;
        ^^^^^^ 
        // TS2532: Object is possibly undefined

    }
}