JS Engine

What do we know/heard about JS Engine?

• Most cases, it uses V8 Engine internally
• It is single threaded and use call-back
• It is an interpreted language

Let's talk about what this engine, we are mentioning.

If I give a plain JS file to a Computer, It can not read or interpret the language. The CPU only understands the 0's and 1's.

To fill this communication gap, we need a translator or interpreter. This is the position the V8 Engine come to play.

This magical engine read our code and interpret it to the CPU.

ECMA-Script Engine

• First JS Engine
• Firefox still using spider monkey

Before 2008, most browser use very basic engines. When V8 came up, it was a significant pivotal time of JS.

V8 was released by Google. While they were developing the Map application, it requires a powerful engine compare to traditional available JS engine.

Inside V8 Engine

• Most popular
• Most usages
• Blazing Fast
• Used by
• Chrome
• Node.js
• Written in C++
• It took the code and do the followings Lexical Analysis
• Parser (Do the tokenizations)
• AST (aka, Abstract Syntax Tree From token, it forms the tree)
• Interpreter (Translate the code to ByCode)
• Profiler (Watcher for the Interpreter and looking for Optimization)
• Compiler (If Profiler point out for some Optimization Required in the Interpreter it )
• Optimization

Interpreter

• Translate and read the code line by line
• Interpreting code means
• Take a set of instructions
• Return answer by doing according to the instructions
• In this case translation is happening on the fly

Compiler

• It does not translate on the fly
• It works ahead of time
• Went through the whole code base
• Convert in a such way, the machine can understand the code
• Example
• Babel (Take modern JS and make them compatible to Old browser)
• Typescript (Superset of JS, compile down to JS)

Interpreter Vs Compiler

• Interpreter
• Quick to get up and running
• Slower for repeated code
• Compiler
• Took little more time to start up compare to the Interpreter
• Can use some optimization mechanism for repeated codebase

JIT Compiler

Stands for Just In Time compiler.

Both the interpreter and compiler has its pro and cons.

Some engineers from google came up with the goodies of them and created the JIT Compiler

V8 and JIT Compiler

In V8, the code is translated to Byte Code by Interpreter. The V8 is intelligent enough to determine if there is an optimization is required. In this case the V8 engine brings the JIT Compiler.

Is Javascript interpreted language?

It depends on the implementation.

Initially JS was the interpreted language. Now with V8 it's the Compiled language.

Same goes for the Python. It could be both, interpreted or compiled.