How Can I Group Objects in a JavaScript Array by a Shared Key?

Grouping Objects in Arrays by Key

In programming, it's often necessary to group objects within an array based on a shared key. Consider the following array of car objects as an example:

const cars = [
    {
        'make': 'audi',
        'model': 'r8',
        'year': '2012'
    },
    {
        'make': 'audi',
        'model': 'rs5',
        'year': '2013'
    },
    {
        'make': 'ford',
        'model': 'mustang',
        'year': '2012'
    },
    {
        'make': 'ford',
        'model': 'fusion',
        'year': '2015'
    },
    {
        'make': 'kia',
        'model': 'optima',
        'year': '2012'
    },
];

The goal is to create a new array where car objects are grouped by make. Using vanilla JavaScript, one approach is to employ the Array#reduce method along with an object:

const result = cars.reduce((r, a) => {
    r[a.make] = r[a.make] || [];
    r[a.make].push(a);
    return r;
}, Object.create(null));

console.log(result);

This code iterates through the cars array and checks if the current make exists as a key in the result object. If not, it adds the make as a key and initializes an empty array. It then pushes the current car object into the corresponding array, effectively grouping objects by their make.

The resulting result object will be structured as follows:

{
    'audi': [
        {
            'model': 'r8',
            'year': '2012'
        },
        {
            'model': 'rs5',
            'year': '2013'
        },
    ],

    'ford': [
        {
            'model': 'mustang',
            'year': '2012'
        },
        {
            'model': 'fusion',
            'year': '2015'
        }
    ],

    'kia': [
        {
            'model': 'optima',
            'year': '2012'
        }
    ]
}

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