Merge arrays inside of array React.js

I have this array:

arr = [[1,2,3,4,5],[3,5,6,8],[3,2,1,5,7,9,10,11],[3,5,6,8,2,1,3,4,6]]

I want it to become

arr = [1,2,3,4,5,3,5,6,8,3,2,1,5,7,9,10,11,3,5,6,8,2,1,3,4,6]

How can I do that?