break array of objects into separate arrays based on a property

Question

Say I have an array like this:

var arr = [
    {type:"orange", title:"First"},
    {type:"orange", title:"Second"},
    {type:"banana", title:"Third"},
    {type:"banana", title:"Fourth"}
];

and I want this to be split up into arrays that have objects which have same type so:

[{type:"orange", title:"First"},
{type:"orange", title:"Second"}]

[{type:"banana", title:"Third"},
{type:"banana", title:"Fourth"}]

But I want to do this generically so not having an if statement that specifies orange or banana

// not like this
for (prop in arr){
    if (arr[prop] === "banana"){
       //add to new array
    }
}

Thoughts? JQuery and Underscore are both options to use.

Answer 1

This is an easy job for Array.reduce(...):

function groupBy(arr, property) {
  return arr.reduce(function(memo, x) {
    if (!memo[x[property]]) { memo[x[property]] = []; }
    memo[x[property]].push(x);
    return memo;
  }, {});
}

var o = groupBy(arr, 'type'); // => {orange:[...], banana:[...]}
o.orange; // => [{"type":"orange","title":"First"},{"type":"orange","title":"Second"}]
o.banana; // => [{"type":"banana","title":"Third"},{"type":"banana","title":"Fourth"}]

Of course, if your target browser(s) do not support ECMAScript 262 5th edition then you'll have to implement "reduce" by yourself, or use a polyfill library, or choose another answer.

[Update] Here's a solution that should work with any version of JavaScript:

function groupBy2(xs, prop) {
  var grouped = {};
  for (var i=0; i<xs.length; i++) {
    var p = xs[i][prop];
    if (!grouped[p]) { grouped[p] = []; }
    grouped[p].push(xs[i]);
  }
  return grouped;
}