String(num) is actually more versatile and consistent than num.toString(). For example, toString() will throw an error if num is null or undefined, while String(null) safely returns "null". Even though it’s not needed here, using String() is more robust and still very readable.

[...] is called the spread syntax. When we write [...str], it spreads the string into an array of its individual characters. This is necessary because reverse() only works on arrays.

Without the spread (just [str]), you'd get an array with the entire string as a single element — not what we want in this case.