Extract object attribute from list of objects in Javascript

Question

I have the following object that I'm receiving from an API:

{
   '2012-12-12': [
       { 'id': 1234,
         'type': 'A' },
       { 'id': 1235,
         'type': 'A' },
       { 'id': 1236,
         'type': 'B' },
    ],
   '2012-12-13': [
       { 'id': 1237,
         'type': 'A' },
       { 'id': 1238,
         'type': 'C' },
       { 'id': 1239,
         'type': 'B' },
    ]
}

Then I want to have another variable named types of type Array that will hold every possible value of the type attribute of each one of the objects. In this case it would be:

types = ['A', 'B', 'C']

I'm trying to have it done in a functional way (I'm using underscore.js) but I'm unable to figure out a way of doing it. Right now I'm using

types = [];
_.each(response, function(arr1, key1) {
    _.each(arr1, function(arr2, key2) {
        types.push(arr2.type);
    });
});
types = _.uniq(types);

But that's very ugly. Can you help me in figuring out a better way of writing this code?

Thanks!

Your code is clear. There is no problem with it in my opinion. — Denys Séguret
– Denys Séguret, Commented Feb 12, 2013 at 21:40
You could keep a separate "typeMap" variable, whose value would start as just a simple object ({}). Then you can use it as a way to keep track of types you've already seen. — Pointy
– Pointy, Commented Feb 12, 2013 at 21:41
@dystroy if the original object is large and the number of types is small, there's some unnecessary work done, but I agree it's probably not really a bad thing at small scale. — Pointy
– Pointy, Commented Feb 12, 2013 at 21:42
@Pointy yes, that would be an optimization, but probably not really useful. — Denys Séguret
– Denys Séguret, Commented Feb 12, 2013 at 21:43
I'd change the name of arr2 as it's actually an object in this case, but that's more developer empathy than functional improvement. — Evan Davis
– Evan Davis, Commented Feb 12, 2013 at 21:45

John Dvorak · Accepted Answer · 2013-02-12 22:18:19Z

5

This should work:

types = _.chain(input) // enable chaining
  .values()            // object to array
  .flatten()           // 2D array to 1D array
  .pluck("type")       // pick one property from each element
  .uniq()              // only the unique values
  .value()             // get an unwrapped array

Fiddle: http://jsfiddle.net/NFSfs/

Of course, you can remove all whitespace if you want to:

types = _.chain(input).values().flatten().pluck("type").uniq().value()

or without chaining:

types = _.uniq(_.pluck(_.flatten(_.values(input)),"type"));

flatten seems to work on objects, even though the documentation clearly states it shouldn't. If you wish to code against implementation, you can leave out the call to values, but I don't recommend that. The implementation could change one day, leaving your code mysteriously broken.

edited Feb 12, 2013 at 22:18

answered Feb 12, 2013 at 21:45

John Dvorak

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the system · Accepted Answer · 2013-02-12 22:36:05Z

1

If you just want shorter code, you could flatten the objects into a single Array, then map that Array.

var types = _.unique(_.map(_.flatten(_.toArray(response)), function(arr) {
    return arr.type;
}));

Here's another version. Mostly just for curiosity's sake.

var types = _.unique(_.pluck(_.reduce(response, _.bind(Function.apply, [].concat), []), "type"));

Here's another one.

var types = _.unique(_.reduce(response, function(acc, arr) {
    return acc.concat(_.pluck(arr,"type"));
}, []));

And another.

var types = _.unique(_.pluck([].concat.apply([], _.toArray(response)), "type"))

edited Feb 12, 2013 at 22:36

answered Feb 12, 2013 at 21:51

the system

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2 Comments

John Dvorak Over a year ago

I don't think you can flatten an object

the system Over a year ago

@JanDvorak: You're right. The source shows that it only performs a type check on the values, not the outer structure, so because each() is used on the outer structure, there's no complaint. And since it creates a new Array instead of modifying the original, the .push()s succeed. But the docs clearly require an array.

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