JetBrain's background indexing is the biggest win for me.
VSCode seems to only see the code in currently open file. If you reference something from a different file, it has no idea what to make of it. This works fine when you import all the types you intend to use (a la typescript), but falls apart for pure js with JSDoc annotations which I prefer.
WebStorm has a daemon running in background that maintains knowledge base of all the symbols defined in entire codebase. This makes all the difference in the world if you like intellisense-style tooling.
Indexing is also one of the things which makes the typing experience very laggy and it can be really really annoying that you can't do anything responsively when a bigger reindexing job kicks in...
But it's a good trade-off most of the time!
You can also pause it and use the IDE just as a more traditional editor, while still having access to scope-selection expand/shrink (cmd-up/dn), refactoring (shift-f6, cmd-alt-v) and fuzzy search in many places in a snappy way...
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