Group objects by property in javascript

Question

Assuming the original list is contained in a variable named list:

_
.chain(list)
.groupBy('type')
.map(function(value, key) {
    return {
        type: key,
        foods: _.pluck(value, 'food')
    }
})
.value();

Without using underscore:

var origArr = [
    {food: 'apple', type: 'fruit'},
    {food: 'potato', type: 'vegetable'},
    {food: 'banana', type: 'fruit'}
];

/*[
    {type: 'fruit', foods: ['apple', 'banana']},
    {type: 'vegetable', foods: ['potato']}
]*/

function transformArr(orig) {
    var newArr = [],
        types = {},
        i, j, cur;
    for (i = 0, j = orig.length; i < j; i++) {
        cur = orig[i];
        if (!(cur.type in types)) {
            types[cur.type] = {type: cur.type, foods: []};
            newArr.push(types[cur.type]);
        }
        types[cur.type].foods.push(cur.food);
    }
    return newArr;
}

console.log(transformArr(origArr));

DEMO: http://jsfiddle.net/ErikE/nSLua/3/

Credit goes to @ErikE for improving/reducing my original code to help with redundancy I had :)

Here is a slightly different but more generic version of @Ian's answer

Caveat: the result is slightly different from the OP requirement, but others like me who stumble on this question might benefit from a more generic answer IMHO

var origArr = [
   {food: 'apple', type: 'fruit'},
   {food: 'potato', type: 'vegetable'},
   {food: 'banana', type: 'fruit'}
];

function groupBy(arr, key) {
        var newArr = [],
            types = {},
            newItem, i, j, cur;
        for (i = 0, j = arr.length; i < j; i++) {
            cur = arr[i];
            if (!(cur[key] in types)) {
                types[cur[key]] = { type: cur[key], data: [] };
                newArr.push(types[cur[key]]);
            }
            types[cur[key]].data.push(cur);
        }
        return newArr;
}

console.log(groupBy(origArr, 'type'));

You can find a jsfiddle here

An ES6 solution to this old question:

Iterate using Array#reduce, and collect the items by group into a Map. Use spread to convert the Map#values back into array:

const data = [
    {food: 'apple', type: 'fruit'},
    {food: 'potato', type: 'vegetable'},
    {food: 'banana', type: 'fruit'},
];

const result = [...data.reduce((hash, { food, type }) => {
  const current = hash.get(type) || { type, foods: [] };
  
  current.foods.push({ food });
  
  return hash.set(type, current);
}, new Map).values()];

console.log(result);

var foods = [
    {food: 'apple', type: 'fruit'},
    {food: 'potato', type: 'vegetable'},
    {food: 'banana', type: 'fruit'}
];

var newFoods = _.chain( foods ).reduce(function( memo, food ) {
  memo[ food.type ] = memo[ food.type ] || [];
  memo[ food.type ].push( food.food );
  return memo;
}, {}).map(function( foods, type ) {
    return {
        type: type,
        foods: foods
    };
}).value();

http://jsbin.com/etaxih/2/edit