JS Object values start with 'undefined'

Question

I am trying to use an array as a key value type scenario and it is working with the exception that every value starts with 'undefined'. I believe this is due to the initial assignment being a += operator however I am not sure how to resolve it.

This is the code stripped of a lot of string concats....

   var phasehtml = {};
        $.each(json, function (i, item) {
            phasehtml[item.Phase] += 'item:'+item.ID;
        });

Basically I am trying to append the string to the appropriate key....

Each key is undefined to begin with, you are then concatenating a string to it, thus getting undefineditem:189 — user400654
– user400654, Commented Apr 1, 2013 at 18:05
@BlueBird Note: phasehtml is an Object. It is not an Array. — ErikE
– ErikE, Commented Apr 1, 2013 at 18:05

Dan Wich · Accepted Answer · 2013-04-01 19:11:57Z

3

You can change the code to only append the ID if there's already IDs:

var phasehtml = {};
$.each(json, function (i, item) {
   // Use the existing value for the phase, or an empty string that we can append to
   var existingValue = (phasehtml.hasOwnProperty(item.Phase) ? phasehtml[item.Phase] : "");
   phasehtml[item.Phase] = existingValue + 'item:' + item.ID; 
});

That's assuming that you want phasehtml to contain an appended lists of the form "item:1item:2" for each phase.

edited Apr 1, 2013 at 19:11

answered Apr 1, 2013 at 18:04

Dan Wich

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7 Comments

Colin DeClue Over a year ago

Another (potentially) cleaner way to write this: phasehtml[item.Phase] = (phaseHtml[item.Phase] || "") + 'item:'+item.ID;

ErikE Over a year ago

This code doesn't work generically, as if (phasehtml[item.Phase]) will fail if the value is "falsy" such as 0. I realize that's not the use case here, but if we're considering use cases, I think it is extremely improbable that the OP does want item:1item:2item:3 as his final result. I know we are all guessing, but I think this guess as to his usage scenario falls short of the mark. He just needs an equal sign. In my opinion a more detailed explanation more about objects and assignments is valuable--but in text, not solely in mis-applied code.

ErikE Over a year ago

@ColinDeClue For what it's worth just keep in mind the "falsy" part of my above comment.

Colin DeClue Over a year ago

@ErikE Of course, but if it starts out undefined, and he's only adding "item:1", etc, to it, then it won't be a problem. But, yes, falsy statements do fail for 0, and this type of thing should never be used for anything that could be a number.

Dan Wich Over a year ago

BlueBird, do take note of Erik's comments if you don't actually intend each phase to have a concatenated list of item IDs. As well as the potential problem with "falsy" values.

|

Saurav Pandit · Accepted Answer · 2013-04-01 18:16:06Z

0

The array you have posted is empty.

var phasehtml = {};

It seems that is the cause the following statement phasehtml[item.Phase] is being evaluated to "undefined".

answered Apr 1, 2013 at 18:16

Saurav Pandit

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Comments

Code Lღver · Accepted Answer · 2013-04-01 18:19:26Z

0

Hmmm,

got the problem.

In your code you are trying to add with that value which is previously not defined that's why this error is occur.

In your code you have not initialize the variable that you are adding.

So try this:

  var phasehtml = {};
    $.each(json, function (i, item) {
        phasehtml[item.Phase] = "";
        phasehtml[item.Phase] += 'item:'+item.ID;
    });

In this first assign some value, here is blank and then use that index of array.

answered Apr 1, 2013 at 18:19

Code Lღver

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