JavaScript, Inline Caching in JavaScript

Explain inline caching in JavaScript.
Answer: Inline caching is a technique used in JavaScript engines to optimize property access and method calls. The main idea behind inline caching is to speed up the repeated access to object properties by caching the information about the object's structure and the location of properties directly in the machine code generated by the Just-In-Time (JIT) compiler. Here's a detailed explanation of how it works:When accessing a property of an object in JavaScript, the engine typically follows these steps:
1. Look up the object to find its hidden class or shape.
2. Find the property within that hidden class or shape.
3. Access the value at the location specified by the hidden class.1. Initial Access: The first time a property is accessed, the JavaScript engine performs the usual lookup steps, which can be relatively slow.
2. Cache the Access: After the first access, the engine stores the information about the hidden class and the property location in an inline cache (IC). This cache is often embedded directly in the machine code.
3. Subsequent Accesses: On subsequent accesses to the same property, the engine uses the cached information to bypass the full lookup process, making property access significantly faster.There are two primary types of inline caches:
1. Monomorphic Inline Cache: This cache stores a single type of shape for an object. It works well if the property access pattern is consistent (i.e., accessing the same property on objects of the same shape).
2. Polymorphic Inline Cache: This cache can store multiple shapes for objects. It's used when the property is accessed on objects of different shapes but still benefits from caching the shapes.- Speed: By avoiding repeated lookups, inline caching reduces the overhead of property access and method calls.
- Efficiency: The inline cache takes advantage of the predictable access patterns in many JavaScript programs.
- Optimization: Inline caches can trigger further optimizations in the JIT compiler, such as inlining functions and eliminating unnecessary checks.Consider the following code:
javascript
function foo(obj) {
return obj.x;
}
let obj1 = { x: 10 };
let obj2 = { x: 20 };
console.log(foo(obj1)); // Initial access, cache is empty
console.log(foo(obj2)); // Subsequent access, using the cache

1. On the first call 'foo(obj1)', the engine performs the full property lookup and caches the location of 'obj1.x'.
2. On the second call 'foo(obj2)', the engine uses the cached information to quickly access 'obj2.x'.- Shape Changes: If the shape of an object changes (e.g., properties are added or deleted), the inline cache may become invalid, requiring a full lookup again.
- Cache Misses: If the property access patterns are highly dynamic, the benefits of inline caching may be reduced due to frequent cache invalidations or misses.
Overall, inline caching is a powerful optimization technique that significantly improves the performance of property access and method calls in JavaScript by leveraging the predictability of these operations in typical JavaScript code.