According to MDN web docs “it is strongly recommended that the assignment operators (+, +=) are used instead of the concat() method.”
According to performance tests, the assignment operators are several times faster.
Sorry, I misread your question.
You can’t use the + operator for concatenation of arrays. The + operator performs a type conversion. It converts all arrays to strings. So the result of
[1,2,3] + [4,5,6]
would not be an array anymore.
Only the concat method or the spread operator are suitable for merging arrays.