What does console.log([1, 2, 3] + [4, 5, 6]) output?

Console output

Why

There is no array addition in JavaScript, so + converts both arrays to strings via toString, which joins elements with commas: "1,2,3" and "4,5,6". Those two strings are then concatenated directly with no separator, producing "1,2,34,5,6". The seam in the middle (34) is where the two arrays meet.