Create 17-JSEngine-ChromeV8.md

Author: HariAcidReign

Notes/17-JSEngine-ChromeV8.md

@@ -0,0 +1,33 @@
+# Episode 17 : JS Engine and Google v8 architecture
+
+#### JS runs literally everywhere from smart watch to robots to browsers because of Javascript Runtime Environment (JRE)
+
+- JRE consists of a JS Engine (❤️ of JRE), set of APIs to connect with outside environment, event loop, Callback queue, Microtask queue etc.
+- JRE is a container that can run JS code. 
+
+- ECMAScript is a governing body of JS. It has set of rules followed by all JS engines like Chakra(Edge), Spidermonkey(Firefox), v8(Chrome)
+- JS Engine is **not a machine**. Its software written in low level languages (eg. C++) that takes in hi-level code in JS and spits out low level machine 
+code 
+
+In all languages, code is compiled either with **interpreter** or with **compiler**. JS used to have only interpreter in old times, but now has **both**
+to compile JS code. 
+
+Interpreter : Takes code and executes line by line. Has no idea what will happen in next line. Very fast.
+Compiler : Code is compiled and an optimized version of same code is formed, and then executed. More efficient
+
+- Code inside JSE passes through 3 steps : **Parsing, Compilation and Execution**
+
+1. **Parsing** - Code is broken down into tokens. In "let a = 7" -> let, a, =, 7 are all tokens. Also we have a **syntax parser** that takes code and converts it
+into an **AST (Abstract Syntax Tree)** which is a JSON with all key values like type, start, end, body etc (looks like package.json but for a line of code in JS. Kinda 
+unimportant)(Check out astexplorer.net -> converts line of code into AST)
+
+2. **Compilation** - JS has something called **Just-in-time(JIT) Compilation - uses both interpreter & compiler**. Also compilation and execution both go hand in hand. 
+The AST from previous step goes to interpreter which converts hi-level code to byte code and moves to execeution. While interpreting, compiler also works hand in hand
+to compile and form optimized code during runtime. 
+
+3. **Execution** - Needs 2 components ie. Memory heap(place where all memory is stored) and Call Stack(same call stack from prev episodes). There is also a *garbage collector.*
+It uses an algo called **Mark and Sweep**. 
+
+Companies use different JS engines and each try to make theirs the best. 
+
+- v8 of Google has Interpreter called *Ignition*, a compiler called *Turbo Fan* and garbage collector called *Orinoco*