How to countBy in JavaScript

The countBy function is one of the functions why people use Lodash in their JavaScript code base. Here I want to give you a brief example on how to implement countBy in vanilla JavaScript without Lodash by just using JavaScript's reduce method.

Let's say we have the following array of objects and we want to count them by property (here color) to get the following output:

const users = [
  { name: 'Jim', color: 'blue' },
  { name: 'Sam', color: 'blue' },
  { name: 'Eddie', color: 'green' },
];

const countByColor = // TODO: implement countBy

console.log(countByColor);

// { blue: 2, green: 1 }

We can use JavaScript's reduce method on an array to iterate over every item:

const countByColor = users.reduce((acc, value) => {
  // TODO: implement countBy

  return acc;
}, {});

We start with an empty object as our accumulator (here acc) for this reduce's callback function. For every iteration of the function, we return the changed (here still unchanged) accumulator. Let's implement countBy:

const usersByColor = users.reduce((acc, value) => {
  if (!acc[value.color]) {
    acc[value.color] = 1;
  } else {
    acc[value.color]++;
  }

  return acc;
}, {});

If the accumulator has no count initialized for currently iterated value's color, we initialize it with the count of 1 allocated in the object whereas the color is the key. If there is a count, we can just increment it by one with the ++ operator.

Essentially we start with an empty object and for every iterated value, we negotiate whether we need to allocate a new count with 1 based on the property (here color) in this object. If not, we increment the count by one, because we already started counting it.