turn array within array into one single array

Question

I know if I have multiple array I can merge them by just doing push like

Array.prototype.push.apply(array1, array2);

but now in my case my array look like this

[[1,2],[a,b]]

not sure if there's a better method than spiting them and then recombine them or not.

I want to achieve [1,2,a,b]