allEqualBy

Checks if all elements in an array are equal, based on the provided mapping function.

• Apply fn to the first element of arr.
• Use Array.prototype.every() to check if fn returns the same value for all elements in the array as it did for the first one.
• Elements in the array are compared using the strict comparison operator, which does not account for NaN self-inequality.

const allEqualBy = (arr, fn) => {
  const eql = fn(arr[0]);
  return arr.every(val => fn(val) === eql);
};

allEqualBy([1.1, 1.2, 1.3], Math.round); // true
allEqualBy([1.1, 1.3, 1.6], Math.round); // false