How does the v8 engine work?
When the V8 engine executes the JS source codes, firstly the parser gets the codes and then constructs a data structure called AST and scope based on it. Next, the bytecode generator runs through the AST and generates the bytecode. Then Ignition compiler reads the bytecode and executes each instruction in turn, performing the operations specified by the bytecode. This allows the code to be executed quickly, even before it is optimized into machine code by the TurboFan compiler. Finally, the TurboFan optimizing compiler converts the bytecode into highly optimized machine code, which can be directly performed by the computer processor. 
The bytecode generated by the V8 engine is not directly executable on the computer’s hardware, because it is a platform-independent representation of the original JavaScript code. However, the bytecode can be interpreted by the Ignition interpreter, which is part of the V8 engine and is capable of executing the bytecode. The Ignition interpreter reads the bytecode and executes it by interpreting each instruction and performing the corresponding operations on the data structures in memory. This allows the JavaScript code to be executed on any platform that has a compatible implementation of the V8 engine, regardless of the underlying hardware architecture.
We should note that although the V8 engine is implemented in C++, it generates optimized machine code directly through the multi-stage compilation without compiling js to C++.
why do we use both an interpreter and a compiler?
The main reason for using the interpreter in addition to the compiler is to improve the overall performance of the JS code execution. the interpreter can execute the bytecode very quickly, but without the optimization that the compiler can provide. The compiler, on the other hand, can generate highly optimized machine code that can be executed very quickly, but the compilation process itself can take a significant amount of time.
By using both of them, the V8 engine is able to balance the trade-off between performance and speed of code execution. The interpreter can be used to execute code quickly during the initial phase of execution, allowing the program to start running immediately. Meanwhile, the compiler can run in the background, continuously analyzing the code and generating optimized machine code, which is used to speed up the execution of frequently executed code paths.
In addition, using an interpreter allows for more efficient use of memory, as the bytecode is typically smaller than the machine code generated by the compiler. This can be particularly important for devices with limited memory, such as mobile devices.
the difference between the interpreter and compiler
An interpreter is a program that reads and executes code line by line. It does not translate the code into another form before executing it. Instead, it interprets each instruction and executes it immediately. Interpreted code is usually slower than compiled code because the interpreter has to perform a lot of work at runtime. A compiler, on the other hand, is a program that translates code from one language into another. It reads the source code and generates machine code that can be executed directly by the computer’s processor. Compiled code is usually faster than interpreted code because the translation from source code to machine code only needs to be performed once, at compile-time.
In the V8 engine, both an interpreter and a compiler are used. The Ignition interpreter generates bytecode from the JavaScript code and executes it line by line. This allows the engine to start executing code quickly, even before the optimized machine code is generated. The TurboFan optimizing compiler, on the other hand, generates highly optimized machine code from the bytecode. This machine code is then executed directly by the computer’s processor, providing the highest possible performance. The use of both an interpreter and a compiler allows the V8 engine to provide a balance between startup time and execution speed.
