JavaScript/ The 'this' keyword

THIS.OBJECT

console.log(this);
//printed
//Window {parent: Window, opener: null, top: Window, length: 4, frames: Window, …}


calculateAge(1985);

function calculateAge(year){
   console.log(2016-year);
   console.log(this);// 'this' printed window things here
}
//in the regular function call "this" is always pointing to window.


var john = {
    name: "John",
    yearOfBirth: 1990,
    calculateAge: function() {
        console.log(this); 
        //printed attached object // {name: "John", yearOfBirth: 1990, calculateAge: ƒ}
        console.log(2016-this.yearOfBirth); //26
        /*
        function innerFunction(){
            console.log(this);
            // printed window {...} obj
            // it is inside of obj, but it's still function!!!!!!!!
        }
        innerFunction();
        */
    }
}
john.calculateAge();
//only in method, it is pointed to the object 


var mike = {
    name: "Mike",
    yearOfBirth: 1984
};

mike.calculateAge = john.calculateAge; 
// It was john's "this", but after "mike" called the method it is changed to "mike's".
//->'this' keyword become a something after the method get called
mike.calculateAge();

 

 

THIS.VARIABLE

- in regular function call : points at the global object(window object, in the browser).
- method call : points to the object that called the method.

"this" is not assingned any value until the function that it is in is actually called. 
:this 키워드는 펑션이 호출되기 전 까지는 아무 의미 없다.