Index of a multidimensional array

APL, 1 byte

⊥

Test it on TryAPL.

J, 2 bytes

#.

Where there's an APL, there's a J! Kind of. Takes dimensions as left arg and index as right arg. "Indexing a multidimensional array is essentially mixed base conversion."

JavaScript (ES6), 34 bytes

(d,a)=>a.reduce((r,i,j)=>r*d[j]+i)

Surely reduce must be better than map.