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Restyle Update 17-JSEngine-ChromeV8.md #12

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HariAcidReign merged 2 commits into main from restyled/patch-1
Jun 29, 2022
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28 changes: 14 additions & 14 deletions Notes/17-JSEngine-ChromeV8.md
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Expand Up @@ -3,31 +3,31 @@
#### 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.
- 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
- 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.
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**
- Code inside JRE 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)
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.
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**.
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.
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*
- v8 of Google has Interpreter called _Ignition_, a compiler called _Turbo Fan_ and garbage collector called _Orinoco_

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