Performance of key lookup in JavaScript object

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The V8 design docs imply lookups will be at least this fast, if not faster:

Most JavaScript engines use a dictionary-like data structure as
storage for object properties – each property access requires a
dynamic lookup to resolve the property’s location in memory. This
approach makes accessing properties in JavaScript typically much
slower than accessing instance variables in programming languages like
Java and Smalltalk. In these languages, instance variables are located
at fixed offsets determined by the compiler due to the fixed object
layout defined by the object’s class. Access is simply a matter of a
memory load or store, often requiring only a single instruction.

To reduce the time required to access JavaScript properties, V8 does
not use dynamic lookup to access properties. Instead, V8 dynamically
creates hidden classes behind the scenes. […] In V8, an object changes
its hidden class when a new property is added.

It sounds like adding a new key might be slightly slower, though, due to the hidden class creation.

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