Group array elements based on a function

While JavaScript has a lot of built-in methods for working with arrays, sometimes you need to craft your own solutions to fit specific requirements. One such requirement is to group array elements based on a specific function or property.
Using Array.prototype.reduce(), one can easily perform grouping operations on an array, using an object as the accumulator. Each value iterated over will determine the key under which it should be stored in the resulting object.
Then, knowing the key, Array.prototype.concat() can be used to add the value to the corresponding array in the object. If the key doesn't exist yet, a new array is created with the value as the first element.
In order to make the function more versatile, it accepts either a function or a property name as the grouping criterion. If a function is provided, it will be called with the value, index, and array as arguments. If a property name is provided, the value will be accessed using that property.

const groupBy = (arr, fn) =>
  arr.reduce((acc, val, i) => {
    const key = typeof fn === 'function' ? fn(val, i, arr) : val[fn];
    acc[key] = (acc[key] || []).concat(arr[i]);
    return acc;
  }, {});

groupBy([6.1, 4.2, 6.3], Math.floor);
// { 4: [4.2], 6: [6.1, 6.3] }
groupBy(['one', 'two', 'three'], 'length');
// { 3: ['one', 'two'], 5: ['three'] }

Then, knowing the key, Array.prototype.concat() can be used to add the value to the corresponding array in the object. If the key doesn't exist yet, a new array is created with the value as the first element.
In order to make the function more versatile, it accepts either a function or a property name as the grouping criterion. If a function is provided, it will be called with the value, index, and array as arguments. If a property name is provided, the value will be accessed using that property.

From 30-seconds-of-code: group-array-elements