JSON Arrays and Javascript

Question

im new to json. Ive created objects similar in structure to the attached json string. I believe what ive created, intentionally, is a nested array object...

  {
   "tracelog":{
      "title":"",
      "phase":"12",
      "users":"",
      "logduration":"",
      "logdurationtype":"undefined",
      "showlogduedate":"no",
      "durationstartfield":"",
      "fields":[
         {
            "atts":[
               {
                  "name":"dfvdfv",
                  "fieldtype":"text",
                  "followedbydate":"no",
                  "datetype":"---",
                  "fieldduration":"---",
                  "fielddurationtype":"undefined",
                  "fielddurationstart":"",
                  "autocalcunits":"none",
                  "validationtype":"none",
                  "fieldvisibleto":"",
                  "fieldaccessibleto":"",
                  "dropdown":""
               }
            ]
         },
         {
            "atts":[
               {
                  "name":"ffff",
                  "fieldtype":"text",
                  "followedbydate":"no",
                  "datetype":"---",
                  "fieldduration":"---",
                  "fielddurationtype":"undefined",
                  "fielddurationstart":"",
                  "autocalcunits":"none",
                  "validationtype":"none",
                  "fieldvisibleto":"",
                  "fieldaccessibleto":"",
                  "dropdown":""
               }
            ]
         }
      ]
   }
}

What i want to do is find the length of the array named "atts". Ive parsed the JSON string as follows...

var obj = jQuery.parseJSON(jsonstring);

... and later call the objects thusly ...

obj.tracelog.fields.length

accurately tells me how many fields there are.

obj.tracelog.fields[0].atts.name

... gives me the name, but throws a console error saying field[0] is undefined. so it works, and it doesnt. while ....

obj.tracelog.fields[0].atts.length

does NOT give me the number of fields in the "atts" array (if it is infact an array - id like it to be) but gives me "undefined".

Can someone point out where im going wrong here?

thanks.

Looking at your object structure, obj.tracelog.fields[0].atts is actually an array. So this could not have worked: obj.tracelog.fields[0].atts.name. It wouldn't have errored out, but it should have returned undefined, not the name. You would have needed to use: obj.tracelog.fields[0].atts[0].name — Lukas S.
– Lukas S., Commented May 22, 2014 at 2:23
You don't need jQuery for this. JSON.parse is available in ES5 compliant hosts, there is a polyfill for those that aren't. atts is an array of length 1, it contains a single member that is an object. Perhaps you want the count of properties on that object? e.g. Object.keys(obj.tracelog.fields[0].atts[0]).length in ES5 compliant hosts. — RobG
– RobG, Commented May 22, 2014 at 3:02

Elliot B. · Accepted Answer · 2014-05-22 02:28:29Z

2

obj.tracelog.fields[0].atts is an array so you must access it like so:

obj.tracelog.fields[0].atts[0].name

Working example

Addressing the latter part of your question:

obj.tracelog.fields[0].atts.length does give you the number of items in the array, which in your example is one object. It seems what you actually want to do is count the number of properties on the first object in that array (i.e., obj.tracelog.fields[0].atts[0]). There is no simple .length style property that will give you the number of properties on an object, but you can loop over them and count like this:

var count = 0;
for (var test in obj.tracelog.fields[0].atts[0]) {
    count++;
}
 alert(count);

edited May 22, 2014 at 2:28

answered May 22, 2014 at 2:22

Elliot B.

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6 Comments

Jerska Over a year ago

Object.keys(your_object).length is also a nice one liner, even if less efficient.

Skinner Over a year ago

Thanks Elliot. Yes, that returns the name, however console still returns "TypeError: obj.tracelog.fields[0] is undefined" - but then the next line returns the name per your advice. AND, "obj.tracelog.fields[0].atts.length returns "1" as opposed to "12" - shouldnt it be 12?

Skinner Over a year ago

Oh, or should each of the elements be in brackets? i will test.

Elliot B. Over a year ago

I updated my answer to elaborate probably at the same time you were writing that comment. obj.tracelog.fields[0].atts.length should return 1 because it is an array with one object in it. What you want to do is count the number of properties on obj.tracelog.fields[0].atts[0] object which I've provided in my example above.

Elliot B. Over a year ago

@Jerska Yeah, that is a really nice one-liner, but it won't work on older browsers.

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