Accessing data in a JavaScript arrray

Question

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Find object by id in array of javascript objects

So I have this:

mapArray=
[{"Title":"crop_rotation","subs":"5,6,7,10"},
{"Title":"cover_crops","subs":"2,5,7,8,9,13,14"},
{"Title":"binary_wetlands","subs":"1,2,3,4,5,6,7,8,9,10,11"}]

I am trying to access the subs value based on the Title. I am trying

listofSubs=mapArray["crop_rotation"]("subs");

I don't get anything returned. What am I missing here? I need to take that list and convert to a string but I assume it will come out as a string since I have the object already parsed? Thanks in advance

score 3 · Accepted Answer · edited May 23 '17 at 11:43

First you have to find the object with the specific title. You have to do this by iterating over the array and comparing each object's Title against your value:

function find(arr, key, value) {
    for(var i = 0, l = arr.length; i < l; i++) {
        if(arr[i][key] === value)) {
            return arr[i];
        }
    }
    // return {}; // if you would want it null-safe
}

Then you can access the corresponding subs property:

var obj = find(mapArray, 'Title', 'crop_rotation');
if(obj) {
    var listofSubs = obj.subs;
}

An explanation for why your code does not work:

mapArray["crop_rotation"]("subs");
//      |-      1      -|-   2  -|

|1| tries to access the property crop_rotation of the object in mapArray. Arrays don't have such properties and in your case no object does. crop_rotation is the value of one of the properties.
|2| is a function call. It tries to call the function referenced by mapArray["crop_rotation"] and passes "subs" as first argument. It throws an error if mapArray["crop_rotation"] is not a function (like in your case).

Further information:

Excellent answer and a piece of code I will put in the arsenal. Thanks for your help Felix. — jeynon, Oct 10 '12 at 19:03
$.each(forMapArray, function(index, value) { if (forMapArray[index]["Title"]=="crop_rotation"){ alert("YES"); doCropRotation(); } }); — jeynon, Oct 10 '12 at 19:40
@jeynon: Yep, for processing each object that fullfils a certain criteria, this makes sense. Note that you can also just do: `if(value["Title"]=="crop_rotation"){...}` — Felix Kling, Oct 10 '12 at 20:31

score 0 · Answer 2 · answered Oct 10 '12 at 18:15

0

alert(mapArray[0].Title) = "crop_rotation"

answered Oct 10 '12 at 18:15

Diodeus - James MacFarlane

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score 0 · Answer 3 · answered Oct 10 '12 at 18:15

What you have is an array of objects there. And arrays are numerically indexed. So to get the value you are looking for:

mapArray;    // returns array of objects
mapArray[0]; // returns first object in the array

// returns the value of the `"subs"` key in the first object in the array
mapArray[0]['subs'];

Note this wont find it by title, you need a bit more logic for that. See Felix Kling's answer to do that,

score 0 · Answer 4 · answered Oct 10 '12 at 18:18

You can' just access list(array) by field of it's element. It's named list(array) for purpose - it's not indexed by anything but position. So you can access list like this mapArray[0]. If want to access it by title You should have structur like this

{ "crop_rotation": ...,
  "cover_crops" :....,
  .... 
}

If your input is fixed that you can't change it you have to pre-process it to fit your needs. So you have to iterate over the array and make dictionary out of it changing "Title" values into keys of your new dictionary.

Accessing data in a JavaScript arrray

4 Answers4