There are some questions about the this direction in setTimeout ().

 var x = 1;
    var o = {
        x: 2,
        y: function() {
            console.log(this.x);
        }
    };
    setTimeout(o.y, 1000); 
    
:1,thiswindow
 var x = 1;
    var o = {
        x: 2,
        y: function() {
            console.log(this.x);
        }
    };
    setTimeout(o.y(), 1000);

the result of execution is: 2

this is pointed to o

.

Why does adding () change the direction of this?

Javascript
Aug.27,2021

indicates the call in parentheses, without parentheses indicates the function body, the first call is window, so the result is 1 and this pointing to window, and the second time, as you typed, the object o is called after parentheses, so of course it points to the object itself, so it is 2. / p >

the previous one is equivalent to 

setTimeout(function() {
    console.log(this.x);
}, 1000);

setTimeout internal this points to window

the latter is equivalent to 

o.y()
setTimeout(undefined, 1000);

o

the yjournal this executed points to o

.

o.y passes a function, which is the syntax of window.setTimeout (fn,times) , indicating that fn is executed after times milliseconds, obviously by window

.

o.y () means to execute the function immediately, which has nothing to do with the timer. At this time, the executor is o

.

o.y is function . Execute
o.y () immediately after 1s. The result is 2

.

1 is a direct call, pointing to the global;
2 is a method call, pointing to the caller of the method.

For more information, please see link

.

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