Javascript: How convert array of objects to object with sorted unique arrays?

Question

Have data that has this kind of structure:

$input = [ { animal: 'cat', name: 'Rocky', value: 1 },
           { animal: 'cat', name: 'Spot',  value: 2 },
           { animal: 'dog', name: 'Spot',  value: 3 } ];

Need fastest possible method for converting to this format:

$output = { animal: [ 'cat', 'dog' ],
            name: [ 'Rocky', 'Spot' ],
            value: [ 1, 2, 3 ] };

The output should have keys equal to each of the keys in each object from the input. And the output values should be arrays with the sorted unique values. I found a few ways to do it using nested loops, but slower than I would like. With 30,000 elements to the input array with 8 keys for each of the objects, the best I have been able to do is 300ms in Chrome. Would like to get down to 100ms. Is there any faster method using a map or reduce?

Answer 1

Yet another way for modern browsers:

$input.reduce(function(acc, obj) {
  Object.keys(obj).forEach(function(k) {
    acc[k] = (acc[k] || []).concat(obj[k])
  })
  return acc
},{})

Answer 2

Here's one way.

$input = [ { animal: 'cat', name: 'Rocky', value: 1 },
           { animal: 'cat', name: 'Spot',  value: 2 },
           { animal: 'dog', name: 'Spot',  value: 3 } ];

$output = {animal:{},name:{},value:{}};

$input.forEach(function(v,i) { 
    $output.animal[v.animal] = 1;
    $output.name[v.name] = 1;
    $output.value[v.value] = 1;
});

$output.animal = Object.keys($output.animal);
$output.name = Object.keys($output.name);
$output.value = Object.keys($output.value);

It prevents having to test each Array every time. You can performance compare to see if it helps.

live example: http://jsfiddle.net/TJVtj/1/

---

If you don't want to hardcode the keys, you can make the solution generic.

var keys = Object.keys($input[0]),
    $output = {};

keys.forEach(function(v) {
    $output[v] = {};
});

$input.forEach(function(v) {
    keys.forEach(function(vv) {
        $output[vv][v[vv]] = 1;
    });
});

keys.forEach(function(v) {
    $output[v] = Object.keys($output[v]);
});

live example: http://jsfiddle.net/TJVtj/2/

Warning. All the values will be strings since they're fetched as object keys.