检查是否存在2d阵列索引

Question

I have a 8x8 2 dimensional array named positions and I'm trying to see if there's a way to check if an index is defined or not. I saw an example using

if (typeof positions[8] === "undefined") alert('undefined');

but when I try it using an index for a row and column like this:

if (typeof positions[8][0] === "undefined") alert('undefined');

I'm getting error:

Uncaught TypeError: Cannot read property '0' of undefined

Is there a way to check if an index for a 2D array exists, not only for one index but for both its dimensions?

Answer 1

The problem is that the row doesn't exist you cannot access elements in it.

A solution is

if ((positions[8]||[])[0] === undefined) { ... }

the idea is that undefined is falsy, thus position[8]||[] will return an empty list if row 8 is not present and the second indexing will not raise an error.

The formal problem of that expression is that in case position[8] is indeed undefined a new temporary empty row is allocated just to try to access it. This could be in theory solved by a smart-enough compiler (and may be it is in some implementation) but another option without this temporary allocation could instead be

if (!position[8] || position[8][0] === undefined) { ... }

where he problem is instead that both position and [8] inside the result are looked up twice (if position is a global this can require a dictionary lookup and, even worse than that, accessing position[8] can potentially change what position will resolve to in next lookup if position is for example a proxy). An even more explicit

{
  let row = position[8];
  if (row === undefined || row[0] === undefined) { ... }
}

should AFAIK solve these problems but is however even more verbose and hard to read and probably a bad idea in most cases.

PS: given the general orientation of Javascript of just returning undefined and keeping running it's indeed surprising that undefined[...] wasn't defined to be undefined itself instead of raising an error.

PPS: this problem of accessing an element in a potentially non-existent array, field in a potentially non-existent object or calling a potentially non-existing function is becoming popular and may be in a few months there will be (or there already is) a proposal about adding something like position[?8]===undefined to mean exactly this kind of somewhat common computation. But as far as I know this didn't make it into Javascript (yet).

Answer 2

You have to check both arrays for elements existence, but this approach is shorter:

if (!(8 in positions && 0 in positions[8])) alert('undefined');

Check more about in operator.