# Group array of JavaScript objects by keys

This fork of [JamieMason's implementation](https://gist.github.com/JamieMason/0566f8412af9fe6a1d470aa1e089a752) changes the `key` parameter to be an array of keys instead of just a single key. This makes it possible to group by multiple properties instead of just one.

## Implementation

```js
const groupBy = keys => array =>
  array.reduce((objectsByKeyValue, obj) => {
    const value = keys.map(key => obj[key]).join('-');
    objectsByKeyValue[value] = (objectsByKeyValue[value] || []).concat(obj);
    return objectsByKeyValue;
  }, {});
```

<details>
  <summary>Click to see TypeScript version</summary>
  
  ```typescript
  /**
   * Group array of objects by given keys
   * @param keys keys to be grouped by
   * @param array objects to be grouped
   * @returns an object with objects in `array` grouped by `keys`
   * @see <https://gist.github.com/mikaello/06a76bca33e5d79cdd80c162d7774e9c>
   */
  const groupBy = <T>(keys: (keyof T)[]) => (array: T[]): Record<string, T[]> =>
    array.reduce((objectsByKeyValue, obj) => {
      const value = keys.map((key) => obj[key]).join('-');
      objectsByKeyValue[value] = (objectsByKeyValue[value] || []).concat(obj);
      return objectsByKeyValue;
    }, {} as Record<string, T[]>);
  ```
  
</details>

## Usage

```js
const cars = [
  { brand: 'Audi', produced: '2016', color: 'black' },
  { brand: 'Audi', produced: '2017', color: 'white' },
  { brand: 'Ford', produced: '2016', color: 'red' },
  { brand: 'Ford', produced: '2016', color: 'white' },
  { brand: 'Peugot', produced: '2018', color: 'white' }
];

const groupByBrand = groupBy(['brand']);
const groupByColor = groupBy(['color']);
const groupByBrandAndYear = groupBy(['brand', 'produced']);

console.log(
  JSON.stringify({
    carsByBrand: groupByBrand(cars),
    carsByColor: groupByColor(cars),
    carsByBrandAndYear: groupByBrandAndYear(cars)
  }, null, 2)
);
```

### Output

```json
{
  "carsByBrand": {
    "Audi": [
      {
        "brand": "Audi",
        "produced": "2016",
        "color": "black"
      },
      {
        "brand": "Audi",
        "produced": "2017",
        "color": "white"
      }
    ],
    "Ford": [
      {
        "brand": "Ford",
        "produced": "2016",
        "color": "red"
      },
      {
        "brand": "Ford",
        "produced": "2016",
        "color": "white"
      }
    ],
    "Peugot": [
      {
        "brand": "Peugot",
        "produced": "2018",
        "color": "white"
      }
    ]
  },
  "carsByColor": {
    "black": [
      {
        "brand": "Audi",
        "produced": "2016",
        "color": "black"
      }
    ],
    "white": [
      {
        "brand": "Audi",
        "produced": "2017",
        "color": "white"
      },
      {
        "brand": "Ford",
        "produced": "2016",
        "color": "white"
      },
      {
        "brand": "Peugot",
        "produced": "2018",
        "color": "white"
      }
    ],
    "red": [
      {
        "brand": "Ford",
        "produced": "2016",
        "color": "red"
      }
    ]
  },
  "carsByBrandAndYear": {
    "Audi-2016": [
      {
        "brand": "Audi",
        "produced": "2016",
        "color": "black"
      }
    ],
    "Audi-2017": [
      {
        "brand": "Audi",
        "produced": "2017",
        "color": "white"
      }
    ],
    "Ford-2016": [
      {
        "brand": "Ford",
        "produced": "2016",
        "color": "red"
      },
      {
        "brand": "Ford",
        "produced": "2016",
        "color": "white"
      }
    ],
    "Peugot-2018": [
      {
        "brand": "Peugot",
        "produced": "2018",
        "color": "white"
      }
    ]
  }
}
```

See [playcode.io](https://playcode.io/334315?tabs=console&script.js&output) for example.