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In some of the projects I'm working on as part of my day job, I need to access data in very large JS objects (on the order of thousands of key-value pairs). I'm trying to improve the efficiency of my code, so I came up with a few questions:
obj.field vs obj[field])
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For objects with thousands of named properties, our implementation indeed uses a hash table (as the question guessed), and that means complexity of Object. keys() is O(n log n). That's because a hash table (of course) stores entries in its own order.
Objects will be used when we need fast access, insertion and removal of an element as objects are comparatively much faster than the arrays in case of insertion and removal.
JavaScript objects are implemented using Hash tables under the hood. One thing about Hashing table is that — Hash tables Retrieve method have O(1), constant time complexity because of the use of a hashing function.
Arrays. Because it takes a single step to access an item of an array via its index, or add/remove an item at the end of an array, the complexity for accessing, pushing or popping a value in an array is O(1).
Javascript objects are actually Hashes, so the complexity is O(1) for all engines.
obj.field is an alias for obj['field'], so they have the same performances.
You can find some JS hashes performance tests here, unfortunately only for your browser engine.
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