JavaScript - Hidden Classes and Inline Caching in JavaScript

Hidden classes and inline caching are internal optimization techniques used by modern JavaScript engines to execute object-related code more efficiently. JavaScript objects are dynamic, meaning properties can be added, removed, or reordered at runtime. This flexibility can slow down execution if the engine treats every object access as unpredictable. To overcome this, engines create hidden structures and reuse access patterns so that property lookups and method calls become much faster during execution.

Understanding Hidden Classes

Hidden classes are internal representations that describe the structure of JavaScript objects. When an object is created, the engine assigns it a hidden class that records which properties the object has and the order in which they were added. If multiple objects are created using the same pattern, the engine assigns them the same hidden class. This allows the engine to treat these objects similarly in memory, making property access more efficient and predictable.

How Hidden Classes Change at Runtime

Hidden classes are not fixed and can change when object structure changes. If a new property is added to an object, the engine creates a new hidden class reflecting the updated structure. Objects that follow the same sequence of property additions will share this new hidden class. However, adding properties in different orders can result in different hidden classes, even if the objects have the same properties. This behavior explains why consistent object creation patterns improve performance.

Inline Caching and Property Access Optimization

Inline caching works alongside hidden classes to speed up property access. When the engine encounters a property access for the first time, it records the hidden class of the object and the location of the property in memory. The next time the same property is accessed on an object with the same hidden class, the engine can skip the lookup process and directly access the value. This significantly reduces the cost of repeated property access operations.

Example Demonstrating Hidden Classes and Inline Caching

Consider the following JavaScript code:

function User(name, age) {
  this.name = name;
  this.age = age;
}

const u1 = new User("Asha", 22);
const u2 = new User("Rahul", 25);

Both objects are created using the same property order, so the engine assigns them the same hidden class. Property access like u1.name and u2.name becomes faster due to inline caching. If a new property is later added to only one object, its hidden class changes, and some optimizations may no longer apply.

Why These Optimizations Matter

Hidden classes and inline caching allow JavaScript engines to handle dynamic objects with near-static performance. They reduce repeated lookup costs and make property access predictable without restricting JavaScript’s flexibility. These optimizations are especially important in large applications where objects are accessed frequently, helping improve execution speed and overall responsiveness.