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I've always thought this was very interesting; specifically, the ability to use objects and functions as keys. [1] [1] https://developer.mozilla.org/en-US/docs

• travisoneill1 8y ago
  Is there a use case for that? I've never run into one.
  • CamTin 8y ago
    As one potential example, it would be useful for implementing something similar to Racket's 'dispatch-rules' (https://docs.racket-lang.org/web-server/dispatch.html#%28form._%28%28lib._web-server%2Fdispatch..rkt%29._dispatch-rules%29%29 https://docs.racket-lang.org/web-server/dispatch.html#%28for...) in JavaScript. Specifically, the second return value from this macro is a procedure that generates URLs given a handler function. This allows one to go both ways in URL-handler mapping (generating URLs from handlers/controllers) instead of just one-way (mapping requests to specific handlers/controllers) as in a conventional router in Rails or Django. Once you learn about this pattern, you start to see it in more places. A slightly different example, I am working on a mail router (something akin to procmail) in Racket, and have used a similar pattern to map from predicates (procedures taking an email and returning true or false depending on certain conditions) to symbols representing those predicates in a DSL. A hash table with procedures for keys lets me do this mapping in both directions should I need to. (Note that I am not actually sure whether dispatch-rules itself uses procedure-keyed dictionaries or hash tables under the hood.)
  • rimunroe 8y ago
    Map is really handy for writing memoization-type functions: const memoize = func => { const cache = new Map(); const memoized = arg => { const cached = cache.get(arg); if (cached !== undefined) { return cached; } const result = func(arg); cache.set(arg, result); return result; }; memoized.clearCache = cache.clear.bind(cache); return memoized; }; (Also, you could use WeakMap instead of Map if you knew the memoized function would only ever receive objects as arguments.)