Javascript/jQuery: Get key by value inside a nested array

Question

var myJSON =     
{"data":
    [{"obj1":"value1",
      "obj2":"value2"},

     {"obj1":"value3",
      "obj2":"value4"},

     {"obj1":"value5",
      "obj2":"value6"}]
};

I've got a multidimensional array similar to above. In the above example I can get value4 by calling data[1].obj2 - the key would be 1 in this case.

My problem: I've got value4 but I don't where it is inside the array (in terms of the key). I know value4 belongs to obj2 and I need to find it's corresponding obj1.

I'm guessing the way to solve this is to figure out what the i should be in data[i] for value4 and then simply call data[i].obj1. But how do I get that i?

This is not a multidimensional array. What you have is a simple array with 3 objects inside — Kenneth
– Kenneth, Commented Nov 10, 2013 at 10:44
It's a bit abstract with this mock data, but in general you should be able to loop through the array and validate for each item if the value in "obj1" is the one you need and in that case get the corresponding "obj2" and break the loop. — GolezTrol
– GolezTrol, Commented Nov 10, 2013 at 10:45

T.J. Crowder · Accepted Answer · 2013-11-10 10:51:19Z

I'm guessing the way to solve this is to figure out what the i should be in data[i] for value4 and then simply call data[i].obj1.

obj2, you mean, not obj1.

But how do I get that i?

You search for it. It's a simple loop.

var i;
for (i = 0; i < myJSON.data.length; ++i) {
    if (myJSON.data[i].obj2 === "value4") {
        // Found it
        break;
    }
}
if (i < myJSON.data.length) {
    // Found it, it's at index `i`
}

Or on a system where you can rely on ES5's forEach (either because the engine has it natively, or because you've added a shim):

var i;
myJSON.data.forEach(function(entry, index) {
    if (entry.obj2 === "value4") {
        i = index;
    }
};
if (typeof i !== "undefined") {
    // Found it
}

You can't stop a forEach early, so that does some unnecessary looping. You could use some:

var i;
myJSON.data.some(function(entry, index) {
    if (entry.obj2 === "value4") {
        i = index;
        return true; // Stops loop
    }
};
if (typeof i !== "undefined") {
    // Found it
}

georg · Accepted Answer · 2013-11-10 11:06:12Z

1

Try this simple recursive function:

find = function(where, what) {
    if (typeof where != "object")
        return null;
    var matches = true;
    for (var p in what)
        matches = matches && (where[p] == what[p]);
    if (matches)
        return where;
    for (var p in where) {
        var found = find(where[p], what);
        if(found)
            return found;
    }
    return null;
}

It works with objects of any depth and allows for multiple search keys. For your example:

var data = {"data":
        [{"obj1":"value1",
            "obj2":"value2"},

            {"obj1":"value3",
                "obj2":"value4"},

            {"obj1":"value5",
                "obj2":"value6"}]
};

result = find(data, {"obj2":"value4"})
console.log(result.obj1) // value3

Another (and nicer) way to do the same is to provide a test predicate for the finder function:

find = function(where, test) {
    if (test(where))
        return where;
    if (typeof where != "object")
        return null;
    for (var p in where) {
        var found = find(where[p], test);
        if (found)
            return found;
    }
    return null;
}

result = find(data, function(x) { return x.obj2 == "value4" });

edited Nov 10, 2013 at 11:06

answered Nov 10, 2013 at 10:53

georg

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3 Comments

Alnitak Over a year ago

interesting approach, but the point is the OP doesn't even know obj2. The desired result in this case would be the list of keys - ['data', 1, 'obj2'].

georg Over a year ago

@Alnitak: yes, the question is a bit unclear... let the OP clarify.

Alnitak Over a year ago

ah, yes - it seems the OP does indeed know obj2 - my bad.

Isaac · Accepted Answer · 2013-11-10 10:49:56Z

0

You'll have to manually trawl through your object:

for(var a=0; a<myJSON.data.length; a++)
{
 for(var b in myJSON.data[a])
 {
  if(myJSON.data[a][b]=="value4")
  {
   console.log("myJSON["+a+"]."+b)
  }
 }
}

answered Nov 10, 2013 at 10:49

Isaac

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Comments

Lorenz Lo Sauer · Accepted Answer · 2013-11-10 11:19:23Z

0

I would recommend to revise the entire data-structure, but assume that this is an abstracted example.

for(var ei in myJSON["data"]){
    if (myJSON["data"][ei]['obj2'] == "value4")){
      //ei contains the key
      //do something & break;
    }
}

Here is one-liner, although slower:

myJSON["data"].map(function(e,i){if(e["obj2"]=="value4")return e;}).filter(Boolean)[0];

or for the key:

myJSON["data"].map(function(e,i){if(e["obj2"]=="value4")return i;}).filter(Number)[0];

Also see T.J. Crowders answer using some - still slow but avoiding unnecessary cycles.

edited Nov 10, 2013 at 11:19

answered Nov 10, 2013 at 11:10

Lorenz Lo Sauer

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Comments

Ben Smith · Accepted Answer · 2015-10-28 11:37:50Z

0

If you are using jQuery then you could use the grep function to perform your search:

var searchValue = "value4";

var result = $.grep(myJSON.data, function (e) { 
        return e.obj2 === searchValue;
    });

console.log(result[0].obj1);

See here for a working example.

edited Oct 28, 2015 at 11:37

answered Oct 11, 2015 at 0:27

Ben Smith

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