Retrieve property value from array literal

Question

I keep getting eluded by how property values should be properly retrieved from array literals.

Object literal:

var obj = {
    "p1": "v1",
    "p2": "v2",
    "p3": "v3"
};

for (var key in obj) {
    if (obj.hasOwnProperty(key)) {
        console.log(key + " -> " + obj[key]);
    }
}

Console log output:

p1 -> v1
p2 -> v2
p3 -> v3

Array literal:

var obj = [
    { "pa":"va1", "pb":"vb1" },
    { "pa":"va2", "pb":"vb2" },
    { "pa":"va3", "pb":"vb3" },
    { "pa":"va4", "pb":"vb4" }
];

for (var key in obj) {
    if (obj.hasOwnProperty(key)) {
        console.log(key + " -> " + obj[key]);
    }
}

Console log output (duh!):

0 -> [object Object]
1 -> [object Object]
2 -> [object Object]
3 -> [object Object]

What I'm trying to do: check if a string matches one of the pa values. If it does, do something.

This would be the logic:

var myString = "a value I'm getting from somewhere else ;)"
if (myString == any of the pa values) {
    // do something
}

Any help appreciated!

You're gonna need another `for` loop over the `obj[key]` object. — gen_Eric, Sep 12 '13 at 15:02
The items in the `array` are `object`s. You have the answer right in your question. — Evan Davis, Sep 12 '13 at 15:02
possible duplicate of [Access / process (nested) objects, arrays or JSON](http://stackoverflow.com/questions/11922383/access-process-nested-objects-arrays-or-json) — Felix Kling, Sep 12 '13 at 15:10
@Mathletics Yes, and that's the fact that kept me from asking it before. I'm glad I did because now I know better — Wallace Sidhrée, Sep 12 '13 at 15:17

Richard Dalton · Accepted Answer · 2013-09-12T15:13:11.587

4

Using the filter method (link contains code to add functionality to older browsers - IE8 and older) and then check the length of the resulting array:

var obj = [
    { "pa":"va1", "pb":"vb1" },
    { "pa":"va2", "pb":"vb2" },
    { "pa":"va3", "pb":"vb3" },
    { "pa":"va4", "pb":"vb4" }
];

var filtered = obj.filter(function(element) { 
    return element.pa === myString;
});

if (filtered.length > 0) {
   // contains string
}

JS Fiddle

edited Sep 12 '13 at 15:13

answered Sep 12 '13 at 15:04

Richard Dalton

35,513
6
73
91

Filter is probably what the best way to do what he wants but be sure that you've poly-filled it if you want support for IE8 and below. – JaredMcAteer Sep 12 '13 at 15:07
Good one! This won't require IE support. – Wallace Sidhrée Sep 12 '13 at 15:15

score 2 · Answer 2 · answered Sep 12 '13 at 15:02

2

obj is not a object as you expect, it's an array of objects. You'll need to iterate the array and then iterate the keys in each array item.

var i, key;

for (i = 0; i < obj.length; i++) {
  for (key in obj[i]) {
    if (obj[i].hasOwnProperty(key) {
      console.log(key + " -> " + obj[i][key]);
    }
  }
}

answered Sep 12 '13 at 15:02

JaredMcAteer

21,688
5
49
65

+1'ing this one because it showed how to get the console log to output something similar to the first one :beer: – Wallace Sidhrée Sep 12 '13 at 15:12

MDEV · Answer 3 · 2013-09-12T15:10:57.587

If you don't need to know the parent, there may be a performance boost in not looping through it all (depending on array and object sizes). This will also remove the need to change your code if another nested level of objects is added.

JSON.stringify(obj).indexOf(':"pa1"')>-1

JSON.stringify() turns your array into a string like this:

"[{"pa":"va1","pb":"vb1"},{"pa":"va2","pb":"vb2"},{"pa":"va3","pb":"vb3"},{"pa":"va4","pb":"vb4"}]"

Which we can then of course just do a normal textual search through

JSON.stringify() is supported for IE8 and above

J E Carter II · Answer 4 · 2013-09-13T15:24:48.100

0

This may be oversimplifying, but if you just want to know if you have a string in mystring that is an item in the array, the array can be addressed as indexed by strings.

Update - I ran this through firebug and discovered I had my notation incorrect for this method to work the way I thought it should - namely as a referable list of objects or values by label.

var obj = [];
obj["val1"] = "pa";
obj["val2"] = "pb";

mystring="val1";

if (obj[mystring]){
   alert ('object exists in array');
}
else{
    alert('object with index ' + mystring + ' does not exist');
}

No looping required, though your array setup has to change to support this sort of "look up" approach.

edited Sep 13 '13 at 15:24

answered Sep 12 '13 at 15:21

J E Carter II

1,436
1
22
39

Thanks for the time you put into this, but I'm not getting why you've inverted `"val1: "pa"`. Property precedes value... Moreover your example is just a simple object literal, not an array (which was part of the problem). But cheers for the contribution! – Wallace Sidhrée Sep 12 '13 at 15:30
You're welcome. The inversion literally turns the problem of finding a value on an object in an array on its head. pa above could be an object unto itself, here I have just a string. By creating the array this way, you create an indexed list of objects so you can look-up objects by name simply by reference. I find this sort of thing useful as an associative entity / hash map sort of arrangement. In my example I just looked to satisfy the "does it exist" question, but you could use this more robustly. – J E Carter II Sep 12 '13 at 15:39
Interesting take on it. I'll be revisiting your method later. Cheers! – Wallace Sidhrée Sep 12 '13 at 15:43

Retrieve property value from array literal

4 Answers4