Since JavaScript has variable-size arrays, multidimensional arrays in V8 and, as I believe, other JS engines are basically arrays of pointers to arrays. It’s a different story in C/C++ which have fixed-size arrays and represent multidimensional arrays with a contiguous chunk of memory.

If you use multidimensional arrays in JS for, say, calculations over matrices, a sane approach might be to store all elements in a single-dimension array and make sure that your code accesses elements in a CPU cache-friendly way. With this approach, you may get significantly better performance.