To find out how many times an array element appears with JavaScript, we use the filter method.

For instance, we write

const countInArray = (array, what) => {
  return array.filter((item) => item == what).length;
};

to define the countInArray function.

In it, we get the count the number of items equal to what by calling array.filter with a callback that returns if item equals to what to return an array with entries with what inside.

Then we get the length property to get the count of what in the array returned by filter.