Function
caculateAge 已存於 Global execution context

caculateAge(1998)

function caculateAge(year){
    console.log(2020-year)
}

// output 22

Global execution context 只有function 宣告，變數則報錯 (Function Declaration)

caculateAge(1998);

function caculateAge(year){
    console.log(2020-year);
}

retirement(1998);

var retirement = function(year){
    console.log(65 - (2020 - year));
}

// output 22
// error Uncaught TypeError: retirement is not a function

variable
未宣告狀況

console.log(age)
// error Uncaught ReferenceError: age is not defined

存於 Global execution context 狀況

console.log(age)
var age = 23;
console.log(age)
// output undefined -> Global execution context
// output 22

最後是我看不懂最久的
要搭配昨天提到的執行堆疊

console.log(age)
var age = 23;
function foo(){
    var age = 65;
    console.log(age);
}
foo();
console.log(age);

// output undefined -> Global execution context
// 65 -> Execution context foo()
// 23 -> Global execution context

一開始真的有點難理解，age 照理說要被賦予成65，但結合昨天學到的堆疊概念，可以知道age在不同堆疊被賦予了不同的值，而執行到 foo()時，Global execution context 上被加了一層 Execution context foo()，直到 foo()執行完後被pop掉。

示意圖
Creation Phase

Global execution context (age、foo())

Execution Phase

Execution context foo() -> 執行到 foo() 時創建
Global execution context

foo()內 console.log(age) 後

Global execution context (age、foo())

其實一直都是大概理解而已，果然程式碼還是要親手打在印出來看才理解比較快

新手練功中，如有錯誤觀念，歡迎指正!
課程 : https://www.udemy.com/course/the-complete-javascript-course/