JAVASCRIPT : for loop array undefined -1 index

Question

var alldivstamp = document.getElementsByClassName("divs");
for(var i = 0; i < array.length; i++){
    if(array[i-1].getAttribute("data") > 1){
        //error here = TypeError: array[(i - 1)] is undefined
    }
}

error output: "TypeError: array[(i - 1)] is undefined"

for(var i = 0; i < array.length; i++){
    if (typeof foo !== 'undefined') {
        if(array[i-1].getAttribute("data") > 1){
            //error here = TypeError: array[(i - 1)] is undefined
        }
    }   
}

this did not fix it

for(var i = 0; i < array.length; i++){
    if (typeof foo !== 'undefined' && i < 1 && i > array.length-1) {
        if(array[i-1].getAttribute("data") > 1){
            //error here = TypeError: array[(i - 1)] is undefined
        }
    }   
}

error here too

undefined value in if statement breaks for loop

here is the data structure

<div class="divs"></div>
<div class="divs"></div>
<div class="divs"></div>
.....

when i=0 it checks for -1 in index
"therefor array index does not exist cousing for loop to stop"

SOLUTION:

for(var i = 0; i < array.length; i++){
    if (i > 0) {
        if(array(i-1).getAttribute("data") > 1){
            //do ...
        }
    }if(i == 0){
        //do ...
    }   
}

success!

error : (TypeError: "x" is (not) "y")

TypeError: "x" is (not) "y"

Examples:
TypeError: "x" is undefined
TypeError: "x" is null
TypeError: "undefined" is not an object
TypeError: "x" is not an object or null
TypeError: "x" is not a symbol

Not exactly related, but you might get unexpected results, when comparing strings to numbers. — Teemu
– Teemu, Commented Dec 25, 2016 at 9:55

Nina Scholz · Accepted Answer · 2016-12-25 10:00:56Z

1

According to document.getElementsByClassName, you are getting an array like object with elments.

You could use the index for an access of the item with

array[i]

and for a property, you could use dot notation, like

array[i].foo

or bracket notation

array[i]['foo']

or a method like getAttribute.

array[i].getAttribute('data')

A valid loop, could be this

array = document.getElementsByClassName("divs");
for (var i = 0; i < array.length; i++){
    if (array[i].getAttribute('data')) { // check for truthyness
        // do something
    }
}

edited Dec 25, 2016 at 10:00

answered Dec 25, 2016 at 9:38

Nina Scholz

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Comments

Kamyar Nazeri · Accepted Answer · 2016-12-25 18:43:15Z

1

your loop does not run. like:

 var i = 0; i < array.length; i++

but run like this:

 var i = 0; i <= array.length; i++

perevent from error do this:

for(var i = 0; i <= array.length; i++){
    var data = array[i];
    if(data){
      if(data.getAttribute("data") > 1){
        // your code
      }
    }
}

or

for(var i = 1 ; i <= array.length + 1; i++){
    var data = array[i - 1];
    if(data){
      if(data.getAttribute("data") > 1){
        // your code
      }
    }
}

edited Dec 25, 2016 at 18:43

Kamyar Nazeri

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answered Dec 25, 2016 at 9:47

Saman Hajizade

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Comments

mrid · Accepted Answer · 2016-12-25 09:40:28Z

0

Indexes in array start from 0. When i=0, array(i-1) equals array(-1) which is undefined. And by the way to get an array value, you use square brackets, not circular brackets . eg. arr[1]

So you should use :

for(var i = 0; i < array.length; i++){
    if(array[i].getAttribute("data") > 1){
        // your code here
    }
}

answered Dec 25, 2016 at 9:40

mrid

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Comments

Naghaveer R · Accepted Answer · 2016-12-25 09:42:14Z

0

For the first iteration i=0, then array[i-1] = array[-1]. this is doesn't exists.

This can be something like this

for(var i = 1; i < array.length; i++){
    if(array[i-1].getAttribute("data") > 1){
        //success
    }
}

Or

for(var i = 0; i < array.length; i++){
    if(array[i].getAttribute("data") > 1){
        //success
    }
}

answered Dec 25, 2016 at 9:42

Naghaveer R

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Comments

Community · Accepted Answer · 2020-06-20 09:12:55Z

0

You DOM selector returns an array based on your "divs" class. That is the array you need to do your dot length for.

UPDATE

You are checking to see if the element has the attribute. You should use the "hasAttribute" method.

Your code....

var alldivstamp = document.getElementsByClassName("divs");
for(var i = 0; i < array.length; i++){
    if(array[i-1].getAttribute("data") > 1){
        //error here = TypeError: array[(i - 1)] is undefined
    }
}

Should be...

var alldivstamp = document.getElementsByClassName("divs");
for(var i = 0; i < alldivstamp.length; i++){
    if(alldivstamp[i].hasAttribute("data") === false){
        //error here = TypeError: array[(i - 1)] is undefined
    }
}

edited Jun 20, 2020 at 9:12

CommunityBot

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answered Dec 25, 2016 at 9:55

andre mcgruder

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Comments

score 0 · Accepted Answer · 2016-12-27 12:07:21Z

0

for(var i = 0; i < array.length; i++){
    if (i > 0) {
        if(array(i-1).getAttribute("data") > 1){
            //do ...
        }
    }if(i == 0){
        //do ...
    }   
}

edited Dec 27, 2016 at 12:07

answered Dec 25, 2016 at 13:03

user7339347

2 Comments

Bill the Lizard Over a year ago

Please use the edit link on your question to add additional information. The Post Answer button should be used only for complete answers to the question. - From Review

user7339347 Over a year ago

this is the solution! error syntax was "TypeError: array[(i - 1)] is undefined"

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