Question

I’m looking for a way (in JavaScript) to collect a set of objects into multiple arrays, where each array contains a certain type of object, and the arrays are stored as values in an associative array, with the keys being the types. For example:

Input:

[<apple>, <cat>, <pear>, <mercedes>, <dog>, <ford>, <orange>]

Output:

{
  'fruit': [<apple>, <pear>, <orange>],
  'animal': [<cat>, <dog>],
  'car': [<mercedes>, <ford>]
}

In ruby, you could do the following:

things_by_type = {}
things.each do |thing|
  (things_by_type[thing.type] ||= []) << thing
end

which is nice and concise.

What’s a good pattern for doing the same thing in JavaScript that’s concise and efficient? I could do something like this, but it’s not as nice:

var thing, things_by_type = {};
for (var i = 0; i < things.length; i++) {
  thing = things[i];
  if(things_by_type[thing.type]) {
    things_by_type[thing.type].push(thing);
  } else {
    things_by_type[thing.type] = [thing];
  }
}

Answer 1

I’m not sure if it’s a good pattern, but it’s similar to your ruby sample:

var things_by_type = {};
for (var i in things) {
  var thing = things[i];
  (things_by_type[thing.type] || (things_by_type[thing.type] = [])).push(thing);
}

And if you can assume Javascript 1.6:

var things_by_type = {};
things.forEach(function(thing) {
  (things_by_type[thing.type] || (things_by_type[thing.type] = [])).push(thing);
})