Use WeakMap instead of mutating data with Symbol properties?

Vega currently assigns Symbol(vega_id) properties to passed-in data. For example:

Instead of mutating the passed-in data, Vega could use a WeakMap to associate a unique identifier with each element in the data set. This could be implemented like so in Tuple.js:

var tupleids = new WeakMap();

function tupleid(t) {
  return tupleids.get(t);
}

function setid(t, id) {
  tupleids.set(t, id);
  return t;
}

By avoiding mutation, Vega could use frozen data, and the API would be cleaner (in my opinion). For example, it would avoid these symbol properties appearing unexpectedly (and nondeterministically) when inspecting data in Observable notebooks, where side-effects are best avoided due to dataflow.

[dantman]

I was just hit by this issue too. I wanted to use a Vega chart in a React component using data from a GraphQL API using the Apollo client. The data is queried with useQuery and useMemo memoizes a reordering of the data. Somewhere along this line the data is frozen, because this data is immutable and it is expected that nothing modifies it so in development at least the data is frozen to ensure this expectation is upheld.

[jheer]

Thanks for the suggestion! We’ll look into it. At first glance this looks good to me, assuming there isn’t a significant performance hit.