V8Verified account

I've always avoided maps because I read long ago that they're the wrong tool for pretty much any job, clearly that's wrong. Where DO they make sense? What are some cases they perform better in? (Alternatively, what made it the right choice for Jest?)

Whenever you need the concept of a lookup table without the need to walk up the prototype chain for missing properties, use a map, not an object.

I wonder if @nodejs could improve performance by doing this

Actually I'm pretty sure the module cache would work way faster with a Map instead. So much userland code relying on `delete require._cache[key]` though.

I think Node 8 was using pretty old V8 with unoptimised Map.

How does the performance of `Map` compare to `Object.create(null)`? I'm curious since I'm assuming the major performance change here was not having to walk up the prototype chain when performing lookups

Jest were doing exactly that, creating an object with null for prototype and there were still performance gains. My guess is because Objects can do a lot, so they come with a lot of overhead, Maps are pretty lightweight, so V8 can optimise them well.

"The performance of Map is much better than Object to implement dictionaries when they contain a large amount of entries". Do you know, currently, how large is "large"? Also, is it impacted by how often the dictionary needs to be mutated?