groupBy array

groupby.js

/** 
 *  PROBLEM: Group an array of objects by a common key value. 
 *  For example - Group cars by color. 
 *	 
 *  Following method also handle if the value is an array. 
 **/
const groupBy = key => array => {
  const addEle = (k, v, o) => o[k] = (o[k] || []).concat(v);

  return array.reduce((output, obj) => {
    [].concat(obj[key]).forEach(v => addEle(v, obj, output));
    return output;
  }, {});
};

const cars = [
  { brand: 'Audi',, color: 'black' },
  { brand: 'Audi', color: ['white', 'black'] },
  { brand: 'Ferarri', color: 'red' },
  { brand: 'Ford', color: 'white' },
  { brand: 'Peugot', color: 'white' }
];

console.log(
  JSON.stringify({
    carsByColor: groupBy('color')(cars)
  }, null, 2)
);


/* Should result following
{
  "black": [
    {"brand": "Audi", "color": "black"},
    {"brand": "Audi", "color": ["white", "black"]}
  ],
  "white": [
    {"brand": "Audi", "color": ["white", "black"]},
    {"brand": "Ford", "color": "white"},
    {"brand": "Peugot","color": "white"}
  ],
  "red": [
    {"brand": "Ferarri", "color": "red"}
  ]
}
*/