hasOne

Checks if an array has only one value matching the given function.

• Use Array.prototype.filter() in combination with fn to find all matching array elements.
• Use Array.prototype.length to check if only one element matches fn.

const hasOne = (arr, fn) => arr.filter(fn).length === 1;

hasOne([1, 2], x => x % 2); // true
hasOne([1, 3], x => x % 2); // false