Why does this alert contain undefined?

Question

This is my code and it gets options from a jsTree checkbox tree list:

var level1, r = [];

for (i = 0, j = data.selected.length; i < j; i++) {
    r[0] = data.instance.get_node(data.selected[i]).text.trim();
    r[1] = data.instance.get_node(data.selected[i]).parents.length;

    if(r[1] == 1) {
       level1 += r[0] + ", ";
       alert(level1);
    }
}

The alert of the above contains undefined along with the correct choices from the checkboxes. Any idea why I am getting undefined in the output?

Please post your HTML code as well, for better understanding. — Nishesh Pratap Singh, May 08 '17 at 16:38
`var level1; var r = [];` And `level1 = r[0]+", ";` try this — Alive to die - Anant, May 08 '17 at 16:40
Your level1 var is undefined.. assign blank string to it so that you can concatenate — Mr. Alien, May 08 '17 at 16:41
`var level1,` is `var level1 = undefined,` so you should be initializing it to an empty string. — epascarello, May 08 '17 at 16:41
No.. do you want to assign a blank array to both vars? In that case it Should be like var a = b = someval so that it assings the same val to both var.. make sure you define both vars first else the second will be global — Mr. Alien, May 08 '17 at 16:43
Props to Mr. Alien - I am suitably embarassed..... `var level1 = ''` DOH!!! Apols all. — Homer_J, May 08 '17 at 16:43
Yeah doing += on something that has no initial value usually breaks. Using var with commas is allowed though. `var i, j, k=1, l='hello',` ... etc — James, May 08 '17 at 16:48

score 1 · Accepted Answer · answered May 08 '17 at 16:42

1

The problem is that level1 is undefined when you first try to use it here:

level1 += r[0]+", ";

That's the same as:

level1 = level1 + r[0]+", ";

The problem is level1 undefined the first iteration through and so the text contains undefined and concatenating it concatenates "undefined" as a string as it is coerced. Initialize it to an empty string:

var level1 = "";

That way it will not be undefined when you access it.

answered May 08 '17 at 16:42

Andrew Li

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@Homer_J and you next question is how to not have a trailing comma. ;) – epascarello May 08 '17 at 17:05
@epascarello I thought the same thing xD – Mr. Alien May 08 '17 at 17:29

score 0 · Answer 2 · edited May 23 '17 at 12:02

0

This

var level1, r = [];

for (i = 0, j = data.selected.length; i < j; i++) {
    r[0] = data.instance.get_node(data.selected[i]).text.trim();
    r[1] = data.instance.get_node(data.selected[i]).parents.length;

    if(r[1] == 1){
       level1 += r[0]+", ";
       alert(level1);
    }
}

Should be

var level1 = '';
   //Or leve1 = []; If you really need it as an array.
var r = [];

for (i = 0, j = data.selected.length; i < j; i++) {
    r[0] = data.instance.get_node(data.selected[i]).text.trim();
    r[1] = data.instance.get_node(data.selected[i]).parents.length;

    if(r[1] === 1){
       // level1[0] += r[0]+", "; If array.
       level1 += r[0]+", ";
       alert(level1);
      // alert(level1[0]); If array.
    }
}

The level1 is undefined since level1, r = [] makes r === [] and level1 === undefined.

So I suggest you explicitly declare both variables as shown in the code block above.

Also I fixed comparison operators, see here why

edited May 23 '17 at 12:02

Community

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answered May 08 '17 at 16:46

ArchNoob

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Thanks ArchNoob - appreciated. String was indeed the issue but, as it happens, the above solves a different issue I was facing elsewhere! – Homer_J May 08 '17 at 16:56
Awesome! @Homer_J – ArchNoob May 08 '17 at 17:07
"Is an array since you declared it as ..." - where do you see that? They explicitly concatenate a string so it's probably a string – Andrew Li May 08 '17 at 17:13
@AndrewLi Ahh, you are right. I mistakenly thought `var some, some1 = []` makes both some & some1 === []. But it actually makes some === undefined and some1 === []. I updated the answer to fit the corrections. Thanks! – ArchNoob May 08 '17 at 17:46

Why does this alert contain undefined?

2 Answers2