Cannot set scalar
It does not appear to be possible to set a scalar 'array'. The zarr specification does apparently allow this with shape=[] and the read case is handled in https://github.com/manzt/zarrita.js/blob/main/packages/indexing/src/get.ts with this code:
return indexer.shape.length === 0 ? unwrap(out.data, 0) : out;
However in https://github.com/manzt/zarrita.js/blob/main/packages/indexing/src/set.ts the zero length shape results in a zero length indexer which means nothing gets written to the array.
Looking a bit more into this, is this related to the fact that the only way to represent a typed value suitable for writing to the array is a TypedArray? Which of course is an array! Without a TypedScalar equivalent, it is not so straightforward to set a scalar value. Setting a zarr array with a length 1 TypedArray works fine but is then represented in zarr as having shape [1].