在學Function前，先對 Object 與 primitive 做個簡單的比較

• Object -> 並非真正被賦值，其代表的是指向記憶體的位置
• primitive -> 真正被賦值

primitive

var a = 10;
var b = a;
a = 15;

console.log(a, b);
// output : 10, 15
// a 最後不等於 b 是因為 primitive 為變數本身被賦值

primitive and object

var obj1 = {
    name : 'eric',
    age : 19
}

var obj2 = obj1; //object
var testPrimitive = obj1.age; // primitive
obj1.age = 20;

console.log(obj1.age, obj2.age, testPrimitive);
// 20, 20, 19
// obj1 & obj2 都指向同記憶體，而 testPrimitive 本身被賦值

Function

• 是 Object 的一種實例
• 可被儲存於變數中
• 可以作為參數傳遞給另一個函數
• 可以做為 function 的回傳值

將函數作為參數傳遞給另一個函數

var years = [1998, 1996, 2005];

// fn : function
function arrayCal(arr, fn){
    var response = [];
    for(var i = 0; i < arr.length; i++){
        response.push(fn(arr[i]));
    }
    return response;
}

function calculateAge(age){
    return 2020 - age;
}

function isFullAge(el){
    return el >= 20;
}

var ages = arrayCal(years, calculateAge);

console.log(ages);
// output : [22, 24, 15]

var checkIsFull = arrayCal(ages, isFullAge);

console.log(checkIsFull);
// output : [true, true, false]

function 做為 function 的回傳值

function interview(job){
    if(job === 'Frontend'){
        return function(name){
            console.log('Hi, ' + name + '!');
            console.log('Is JavaScript your main language?');
        }
    }else if(job === 'Backend'){
        return function(name){
            console.log('Good morning, ' + name + '!');
            console.log('Can you please explain what MVC is?');
        }
    }else{
        return function(name){
            console.log('Sorry, ' + name + '.');
            console.log('We are not looking for a ' + job);
        }
            
    }
}

// assign the function to variable and call it
var frontEndInterview = interview('Frontend');
frontEndInterview('sui');
// output : Hi, sui!
// output : Is JavaScript your main language?

var backEndInterview = interview('Backend');
backEndInterview('ken');
// output : Hi, ken!
// output : Can you please explain what MVC is?


// without assign to variable
interview('Teacher')('jay')
// output : Sorry, jay.
// output : We are not looking for a Teacher

IIFEs

除了常見的 Function Statement 和 Function Expression
還有一種寫法 IIFEs (Immediately Invoked Functions Expressions)
也就是立即調用且不需呼叫
不帶參數

(function() {
    var n = Math.random() * 10;
    console.log(n);
    console.log(n > 5);
})();

// output : 3.362294415982372
// output : false

帶參數

(function(n) {
    console.log(n);
    console.log(n > 5);
})(6);

// output : 6
// output : true

都是實作，雖然蠻基礎的，但感覺又進步了一點

課程 : https://www.udemy.com/course/the-complete-javascript-course/
來源 : https://pjchender.blogspot.com/2016/05/javascriptiifesimmediately-invoked.html