Checking for undefined causes undefined error

Question

Trying to check if some variables contain things, because they throw errors and break the ajax function when they don't. Problem is that just checking if data[2][0] contains something causes the following error:

Uncaught TypeError: Cannot read property '0' of undefined

I'd very much prefer not to check this in a previous stage. How do I check if data[2][0] is defined, without causing the actual checking to break my js?

Code:

//ajax ^
success: function(data){
    var xp = data[0][0]; //Contains a string
    var yp = data[1][0]; //Contains a string
    var zp = data[2][0]; //Is not set, fails here

    if(xp === ''){ //Tried using null & undefined here aswell
       //Do nothing   
    } else {
        var one = data[0][0];
        var oneH = data[0][1];
        var oneS = data[0][2];
    }
    if(yp === ''){
        //Do nothing
    } else {
        var two = data[1][0];
        var twoH = data[1][1];
        var twoS = data[1][2];
    }
    if(zp === ''){
        //Do nothing  
    } else {
        var three = data[2][0];
        var threeH = data[2][1];
        var threeS = data[2][2];
    }
//ajax continues v

Any help will be much appreciated.

_Uncaught TypeError: Cannot read property '0' of undefined_ means that `data[2]` isn't defined, and maybe even that `data` isn't defined. — putvande, Aug 30 '16 at 20:25
@putvande Yes, I know it isn't defined. `data[0]` and `data[1]` are both defined, they contain a link each. It's when they're undefined the site breaks, which is why I need to check if they are before using them — Algernop K., Aug 30 '16 at 20:27
If there's a possibility that `data[x]` might be undefined then you have to test that *before* you can try to use `data[x][y]`. (And test with `=== undefined`, not `=== ''`.) But you can still do the test in one line: `if (data[x] === undefined || data[x][y] === undefined) {...}`. — nnnnnn, Aug 30 '16 at 20:33
@nnnnnn Thank you very much. I would have thought that `var zp = data[2][0];` would be defined similar to `var zp = '';`. Experimenting with this worked really well. — Algernop K., Aug 30 '16 at 20:45
JS has to evaluate the `data[2][0]` part before any assignment to `zp` can take place, and it has to evaluate `data[2]` before it can get to the `[0]`. So if `data[2]` is undefined then trying to use `data[2][0]` is basically saying "give me the first element of an array that doesn't exist", and that gives you an error... — nnnnnn, Aug 30 '16 at 20:57
@nnnnnn Got it. Mind putting it in an answer so that I can commend you for helping me? — Algernop K., Aug 30 '16 at 21:02
I kind of don't want to post an answer because I feel like this is a duplicate of [this question](http://stackoverflow.com/questions/2631001/javascript-test-for-existence-of-nested-object-key) (although that question is talking more generally about objects rather than arrays it is conceptually the same thing). — nnnnnn, Aug 30 '16 at 23:55

score 2 · Answer 1 · answered Aug 30 '16 at 20:29

2

You should check if a variable is undefined using typeof, not with an equality check against '':

if (typeof myVar === 'undefined')

You can also check if a variable is an array using Array.isArray(myVar)

answered Aug 30 '16 at 20:29

Rob M.

35,491
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@nnnnnn True, but their question was "checking for undefined causes undefined error", and that is because they are improperly checking for `undefined` AFAICT – Rob M. Aug 30 '16 at 20:38
I tried `typeof`. What you see up there is the result of an hour or so of experimenting. – Algernop K. Aug 30 '16 at 20:39

score 1 · Answer 2 · answered Aug 30 '16 at 20:27

1

I would check to see if data[2] exists first, and then redefine zp if it does.

var zp = data[2];
if (zp) zp = zp[0];

answered Aug 30 '16 at 20:27

larz

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Checking for undefined causes undefined error

2 Answers2