# 執行環境與詞彙環境

## Syntax parsers 語法解析器
A program that reads your code and determines what it does and if its grammar is valid.
Your code isn't magic. Someone else wrote a program to translate it for the computer.

## Execution contexts 執行環境
- A wrapper to help manage the code that is running.
- There are lost sof lexical environments. Which one is currently running is managed via execution contexts. It can contain things beyond what you've written in your code.

## Lexical environment 詞彙環境
- Where something sits physically in the code you write
- "Lexical" means 'having to do with words or grammar'. A lexical environment exists in programming languages in which where you write something is important.
- 如果提到這個，意思是在討論它被寫在哪裡? 它的周圍環境是甚麼?

## Name/value Pair
- A name which maps to a unique value
- The name may be defined more than once, but only can have one value in any given context. That value may be more name/value pairs.
- 

## Object
- A collection of name value paris
- The simplest definition when talking about **Javascript**
- 
- 

## The global environment and the global object
### Global、this 是 Javascript 引擎自動幫你生成的
- Glogal means "Not inside a Function"
- Javascipt 的 Global 是 window，node 的 global 是別的東西。
- outer environment
- 

## The execution context: Creation and hoisting
Execution Context 被分為兩階段創造，第一階段是創造(creation)階段，這個階段會產生 Global Object, 'this', Outer Environment 最後會 Steup memory space for Variables and Functions(這就是 Hoisting)。
表示在執行程式碼之前已經為變數與函數在記憶體中創造空間，所以當程式被逐行執行時，它可以找到它們。
不過變數的情況有點不同，變數都會先被指定為 "undefined"，然後在執行的時候才會被附值，而函式則會被完整儲存起來。

## Undefined
- Javascript 在創造執行環境的第一階段時會為變數設定初始值 `undefined`，如果是一個未曾被設定為初始值的東西則會得到錯誤訊息 `Uncaught ReferenceE: something is not defined`，因為它沒辦法在已經創造的記憶體之中找到這個東西。
- `undefined` 是一個特殊關鍵字，它是一種值，會是創造階段的變數之預設值。
- 永遠不要設定任何東西為 `undefined`，將會有幫助你除錯，只要一看到 `undefined` 時就可以理解為不曾為其附值

## Execution Context Runs code
(Execution phase 執行階段)
逐行執行

## Single threaded & synchronous execution
- (單執行緒、同步執行)，Javascript 是單執行緒、同步執行的
- Single threaded
- One command at at a time
- Under the hood of the browser, maybe not，但從我們的角度來說 Javascript 是單執行緒的
- Synchronous execution
- One at a time
- And in order

## Invocation
Running a function
In javascript, by using parenthesis ()

## Variable environment
Where the variables live. And how they relatve to each other in memory

## scope chain
- Javascript 會依照 Lexical environment 來尋找變數 (此例是尋找變數)，從該 Execution contexts 開始往外面找 outer reference
- 每個 Execution contexts 都會有 outer reference (此例的 function a 的 outer reference 是 global)
- 與函式呼叫的位置沒有關係。

function a(){
    console.log(myVar) // 1
}
function b(){
    var myVar = 2
    a()
}
var myVar = 1
b()

## Scope
- Where a variable is available in your code
- And if it's truly the same variable, or a new copy

## let
- Can be use instead of var
- Allow JavaSciprt engine to use block scoping
- During the execution phase where it's created, the variable is still placed into memory and set to `undefined`. Hoever, you're **NOT** allowed to use let until the lin of code is run.
- When `let` is declared inside the block, it's only available inside that block at that period of time for the running code.

## Asynchoronous
more than one at a time

JavaScript 會先執行 execution stack 的 event，等到完成之後才會去執行 event queue，等到結束時，會持續查看 event queue 的迴圈，這個過程稱為 continuous check。
在 Javascript 提到的非同步其實是在 Javascript 外的引擎執行的，Javascript 本身是同步的。

###### tags: `WierdJavascript`