When JavaScript code gets loaded, the engine has to parse it. This parsing breaks down into two key phases: lexical analysis and syntax analysis. Lexical analysis chops the code into tokens, while syntax analysis takes those tokens and builds them into a syntax tree, which is technically called an Abstract Syntax Tree (AST).   After the AST is set up, it gets converted into an intermediate form called bytecode. Bytecode is a low-level instruction set that the engine understands but isn’t directly executable by the CPU. In the V8 engine, this conversion is handled by an interpreter named "Ignition."   For SpiderMonkey (that’s the engine used in Firefox), the bytecode is first run by an interpreter, which helps in achieving a faster startup. Meanwhile, the V8 engine might decide to go for baseline Just-In-Time (JIT) compilation using "TurboFan." This step converts the bytecode into a binary form that can be executed quickly, aiming for both speed and efficiency.   Both V8 and SpiderMonkey keep an eye on how the code is running. During this profiling stage, they identify the hot code paths—sections of code that get executed over and over again. The data collected during this phase helps in further optimizations, which is crucial for speeding up future runs.   Based on what the profiling data shows, the engines decide which parts of the code need some fine-tuning. In V8, "TurboFan" handles the optimization of these hot paths. SpiderMonkey does something similar using its optimizing JIT compiler called "IonMonkey." Both engines use advanced techniques like inlining, constant folding, and loop unrolling to squeeze out better performance.   Sometimes, optimized code can backfire, leading to unexpected results. When the engine realizes its assumptions about the code were off, it deoptimizes, which means it rolls back to a less optimized version. V8 uses a mechanism called "Bailout" for this, while SpiderMonkey has its own "Bailout" operations to deal with these situations.   Managing memory efficiently is key to keeping performance up. Both engines have advanced garbage collection systems to reclaim memory that’s no longer in use. V8 features the "Orinoco garbage collector," a concurrent, low-latency system. On the flip side, SpiderMonkey uses the "Generational GC," which groups objects by age and cleans them up accordingly.   To speed up how methods are dispatched, both engines make use of inline caching. This technique involves storing the location of methods or properties that are frequently accessed. V8 takes this a step further with "Polymorphic Inline Caches" (PICs) to handle multiple types of objects in the cache. SpiderMonkey does something similar, but they call it "Shape-based Inline Caching."   To keep runtime performance high without bogging down the main execution thread, both engines use background threads to compile code. V8’s "TurboFan" and "Orinoco" handle different parts of their processes in parallel. Likewise, SpiderMonkey taps into background threads in their "IonMonkey" compiler to manage more costly operations.