The purpose of using hidden classes in V8 is to optimize property access time on JavaScript objects. V8 is a JavaScript engine developed by Google that compiles JavaScript directly to machine code for execution, rather than using an intermediate bytecode or another form of interpretation. This direct compilation contributes to the high performance of JavaScript execution in environments like Google Chrome and Node.js.
In dynamically typed languages like JavaScript, objects can have properties added and removed at runtime, and their types can change, which makes property access inherently slower compared to statically typed languages where the memory layout of objects is fixed at compile time. To mitigate this, V8 employs a concept known as hidden classes (also referred to as maps or shapes in other JavaScript engines).
Hidden classes are created at runtime and are used to track the shape or layout of objects. When an object is instantiated, V8 creates a hidden class that maps the properties of the object to fixed offsets in memory. As properties are added to the object, V8 creates new hidden classes that include the offsets for the new properties, and transitions are made from the old hidden class to the new one. This allows V8 to use these offsets to access properties quickly, similar to how it would be done in a statically typed language, thus improving property access times significantly.
The transitions between hidden classes are also important because they allow for hidden classes to be shared among similar objects. If two objects are created with the same properties in the same order, they can share a hidden class, which means that the engine can optimize property access even further by using inline caching. Inline caching is a technique where the engine makes an optimistic assumption that the hidden class of an object will not change, allowing it to bypass the hidden class lookup and directly access the property using its offset.
However, if properties are added to objects in different orders, different hidden classes are created, and the objects cannot share the same optimizations. This is why the order in which properties are added to objects in JavaScript can affect performance.
In summary, hidden classes in V8 serve to optimize property access by providing a mechanism to transition from the slower, dictionary-like property access of dynamic languages to a more efficient, offset-based access that can be optimized similarly to property access in statically typed languages[1][2][3][4][5][6].
