Question

I’m using underscore.js, of course any solution is fine, but I have an object that looks like this:

{ '4f871d4967e04': 
   [ { _id: 4f871d4adaf6fa492f000001,
       product_id: 4f871d43daf6fa4e2f000002,
       width: '300',
       height: 300,
       group: '4f871d4967e04' },
     { _id: 4f871d4adaf6fa492f000004,
       product_id: 4f871d43daf6fa4e2f000002,
       width: '150',
       height: 150,
       group: '4f871d4967e04' },
     { _id: 4f871d4bdaf6fa492f000007,
       product_id: 4f871d43daf6fa4e2f000002,
       width: '100',
       height: 100,
       group: '4f871d4967e04' },
     { _id: 4f871d4bdaf6fa492f00000a,
       product_id: 4f871d43daf6fa4e2f000002,
       width: '75',
       height: 75,
       group: '4f871d4967e04' } ],
  '4f871d51200de': 
   [ { _id: 4f871d51daf6faf42e000001,
       product_id: 4f871d43daf6fa4e2f000002,
       width: '300',
       height: 300,
       group: '4f871d51200de' },
     { _id: 4f871d52daf6faf42e000004,
       product_id: 4f871d43daf6fa4e2f000002,
       width: '150',
       height: 150,
       group: '4f871d51200de' },
     { _id: 4f871d53daf6faf42e000007,
       product_id: 4f871d43daf6fa4e2f000002,
       width: '100',
       height: 100,
       group: '4f871d51200de' },
     { _id: 4f871d53daf6faf42e00000a,
       product_id: 4f871d43daf6fa4e2f000002,
       width: '75',
       height: 75,
       group: '4f871d51200de' } ] }

I’d like to turn it to this:

{"4f871d4967e04" : {
 "300" : {
    _id : '',
  }
 "150" : {
    _id : '',
  }
 "100" : {
    _id : '',
  }
 "75" : {
    _id : '',
  }
}, "4f871d4967e04" : {
 "300" : {
    _id : '',
  }
 "150" : {
    _id : '',
  }
 "100" : {
    _id : '',
  }
 "75" : {
    _id : '',
  }
}

This way I can iterate through each “group” and simply use this["300"] to get the size that I want. I’m iterating through this in Handlebar’s templates, so I’m limited to what conditions I can check for.

Any suggestions would be really helpful.

Thank you!

Answer 1

The obvious plain-jane JavaScript approach would probably be the quickest:

var want = { };
var k, i, o;
for(k in data) {
    want[k] = { };
    for(i = 0; i < data[k].length; ++i) {
        o = data[k][i];
        want[k][o.width] = o;
    }
}

Simply loop through the main object (data) key by key and use an inner loop to rearrange the arrays into width→object mapping tables.

Anything you come up with will probably be that nested loop in disguise. The Underscore version would look something like this:

var want = _(data).reduce(function(outer, v, k) {
    outer[k] = _(v).reduce(function(inner, o) {
        inner[o.width] = o;
        return inner;
    }, { });
    return outer;
}, { });

That’s pretty much the original nested for loop set up with reduce in place of the loops. You’re pretty much limited to using reduce here as most Underscore functions really want to turn everything into an array. You could use each:

var want = { };
_(data).each(function(v, k) {
    var inner = { };
    _(v).each(function(o) {
        inner[o.width] = o;
    });
    want[k] = inner;
});

But that’s the same as the reduce approach with temporary variables. You could probably come up with other approaches but they’ll probably just be more variations on this theme.

Demo: http://jsfiddle.net/ambiguous/ffcUC/2/

---

If you want to send your data into a Handlebars template like:

{{#each images}}<img src="../{{this.300._id}}" />{{/each}}

Then you only want the values from what the above produces. But in this case, you can adjust your loops to save some work:

var want = [ ];
var k, i, o;
for(k in data) {
    o = { };
    for(i = 0; i < data[k].length; ++i) {
        o[data[k][i].width] = data[k][i];
    }
    want.push(o);
}
return want;

Or switch to reduce inside a map:

var index_by_height = function(o) {
    return _(o).reduce(function(memo, i) {
        memo[i.height] = i;
        return memo;
    }, { });
};
return _.chain(data)
        .values()
        .map(index_by_height)
        .value();

Demo: http://jsfiddle.net/ambiguous/tsx7z/