Tricky Javascript: how to retrieve value from json dict inside array inside dict :)

Question

I've got a json file included in my javascript that defines a variable that looks like this:

var places_json = {
  "count": 13476,
  "places": [{
      "area": "London",
      "county": "STS",
      "lat": 52.300151820000004,
      "lon": -2.36665606,
      "code": "7567",
      "id": 1
    },
    {
      "area": "Sheffield",
      "county": "STS",
      "lat": 51.33648680000003,
      "lon": 0.98861179000000001,
      "code": "9919",
      "id": 6
    },
    {
      "area": "Huxton",
      "county": "STS",
      "lat": 53.27483902,
      "lon": -1.0146250700000001,
      "code": "9953",
      "id": 11
    },
  ]
}

And I want to retrieve the value of area for the entry whose id is 11, using Javascript.

Does anyone know how to do this? It's too hard for me - my Javascript is not that advanced. If I knew the array index of the entry that I need, then I could do:

var entry = json_places.places[i]

but I don't know it, unfortunately, just the value of id (which is unique).

Thanks in advance.

No - just javascript (though I can add one if it's impossible to do it any other way) — AP257
– AP257, Commented Feb 3, 2010 at 22:31
It's definitely possible, as you can see in the answers below. I was just asking so that I could provide an answer that uses the library instead of generic JS — adamJLev
– adamJLev, Commented Feb 3, 2010 at 22:36

ChaosPandion · Accepted Answer · 2010-02-03 22:37:38Z

3

function findAreaById(placesArray, id) {    
    var place = null;
    for (var i = 0, length = placesArray.length; i < length; i++) {
        place = placesArray[i]; 
        if (place.id === id) {
            return place.area;
        }
    }
    return null;
}

edited Feb 3, 2010 at 22:37

answered Feb 3, 2010 at 22:32

ChaosPandion

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Comments

Andrew Moore · Accepted Answer · 2010-02-04 00:07:18Z

1

Simply loop through your places to find the proper item.

var area = null;
for(var i = 0; i < json_places.places.length; i++) {
  if(json_places.places[i].id == 11) {
    area = json_places.places[i].area;
    break;
  }
}

alert(area);

If you want a function to do it for you, you can run the following:

json_places.get_place = function(id) {
   return (function(id) {
             for(var i = 0; i < this.places.length; i++) {
               if(this.places[i].id == id) {
                 return this.places[i];
               }
             }

             return null;
           }).apply(json_places, id);
}

Then you can simply do the following:

alert(json_places.get_place(11).area);

edited Feb 4, 2010 at 0:07

answered Feb 3, 2010 at 22:32

Andrew Moore

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7 Comments

Anthony Mills Over a year ago

Use for in for hashes, not arrays. It's a bad habit. :)

Andrew Moore Over a year ago

@Anthony Mills: There is nothing wrong with using for in for arrays. Section 12.6.4 of ECMA-262 states that any object whose [[Enumerable]] attribute is true can be used in such loop. An array qualifies.

AP257 Over a year ago

Cheers everyone for all the answers. Picking this one because it worked perfectly first time & for the combination of inline code & function. Thanks!

James Over a year ago

@Andrew, using for...in works, yes, but it's dead slow - blogs.sun.com/greimer/resource/loop-test.html - and so is generally accepted as a bad practice.

James Over a year ago

@Andrew, also, for...in exposes anything added to Array.prototype or to the Array instance.

|

Anthony Mills · Accepted Answer · 2010-02-03 22:39:08Z

0

function get_area(places_json, id) {
    for (var i = 0; i < places_json.places.length; i++) {
        if (places_json.places[i].id === id) {
            return places_json.places[i].area;
        }
    }
}

Also, if you know you have JavaScript 1.5 available (or you don't mind pasting in the code at https://developer.mozilla.org/en/Core_JavaScript_1.5_Reference/Objects/Array/filter ...), and especially if you know LINQ:

function get_area(places_json, id) {
    return places_json.places.filter(function (place) { place.id === id; })[0].area;
}

edited Feb 3, 2010 at 22:39

answered Feb 3, 2010 at 22:32

Anthony Mills

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Comments

karim79 · Accepted Answer · 2010-02-03 22:40:19Z

0

Using jQuery (even though this question has been sufficiently answered):

var area = null;
$.each(places_json.places, function() {
    if(this.id == 11) {
        alert("Area is " + this.area);
        area = this.area;
        return false;
    }
});

See:

http://api.jquery.com/jQuery.each/

answered Feb 3, 2010 at 22:40

karim79

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