Find the longest element in a JavaScript array

I've often found a need to find the longest element in an array of strings or arrays. It has come especially handy when solving coding challenges. And it's an easy problem to solve, too.
All you need to do is create a function that takes any number of iterable objects or objects with a length property and returns the longest one. This can be done using Array.prototype.reduce() to compare the length of objects.
If multiple objects have the same length, the first one will be returned. If no arguments are provided, the function will return undefined.

const longestItem = (...vals) =>
  vals.reduce((a, x) => (x.length > a.length ? x : a));

longestItem('this', 'is', 'a', 'testcase'); // 'testcase'
longestItem(...['a', 'ab', 'abc']); // 'abc'
longestItem(...['a', 'ab', 'abc'], 'abcd'); // 'abcd'
longestItem([1, 2, 3], [1, 2], [1, 2, 3, 4, 5]); // [1, 2, 3, 4, 5]
longestItem([1, 2, 3], 'foobar'); // 'foobar'

If multiple objects have the same length, the first one will be returned. If no arguments are provided, the function will return undefined.

From 30-seconds-of-code: longest-array-element