JavaScript - JavaScript Engine Architecture

JavaScript engine architecture refers to the internal structure and components responsible for executing JavaScript code. A JavaScript engine is not a single block but a combination of subsystems that work together to read source code, convert it into executable instructions, manage memory, and control execution flow. This architecture allows JavaScript to run efficiently across different platforms while following the same language rules.

The first major component of a JavaScript engine is the parser. The parser reads the JavaScript source code and breaks it into tokens, then converts those tokens into a structured representation known as an abstract syntax tree. This structure represents the logical layout of the code and helps the engine understand how statements, expressions, and functions are connected before execution begins.

Once parsing is complete, the engine moves into the compilation phase. Instead of directly executing source code, modern JavaScript engines convert the abstract syntax tree into intermediate code or bytecode. This compiled form is easier and faster for the engine to execute. Some engines further optimize this code at runtime by analyzing how frequently certain parts of the code are executed.

The execution component of the engine is responsible for running the compiled code. It manages execution contexts, updates the call stack, resolves variable access, and performs operations defined by the program. During execution, the engine continuously interacts with memory systems to read and write values, ensuring that instructions are executed in the correct order.

Memory management is another critical part of JavaScript engine architecture. The engine allocates memory for variables, objects, and functions and tracks their usage throughout execution. Garbage collection mechanisms are integrated into the engine to reclaim memory that is no longer reachable. This integration allows memory cleanup to occur automatically without interrupting normal program behavior.

Modern JavaScript engines also include optimization and de-optimization subsystems. These systems monitor code execution patterns and apply performance improvements when code behaves predictably. If assumptions made during optimization become invalid, the engine safely reverts to less optimized execution paths. This adaptive behavior allows JavaScript engines to balance speed and correctness during runtime.