javascript - js this question

Question

var foo = {
  bar: function () {
    alert(this);
  }
};
 
foo.bar(); // Reference, OK => foo      (1)
(foo.bar)(); // Reference, OK => foo    (2)
 
(foo.bar = foo.bar)(); // global?       (3)

Can you explain why the results of (2) and (3) are different?

Answer 1

var temp =foo.bar=foo.bar;
temp();
Continuous assignment
Understand?
Because this is not called as an object method but as a function call, so this points to undefined, and in non-strict mode undefined points to the global world.

Answer 2

This is defined by the context during execution, so naturally whoever calls this is who

The call in (2) is foo.bar, then this this has found the foo object;

When calling (3), there is an assignment first, and foo.bar is assigned to foo.bar, so that it becomes a global variable. This function is called globally, so of course it points to undefined (non-strict mode in the browser environment).

Answer 3

Judging from the code comments, the questioner may just not understand (3). Here is the explanation:
foo.bar = foo.barThe expression returns an anonymous function, that is, function( ) {alert(this);}. foo.bar = foo.bar表达式返回的是匿名函数，即function() {alert(this);}。
所以再调用相当于该匿名函数的自调用，即(function() {alert(this);})();So the call is equivalent to the self-call of the anonymous function, that is, (function() {alert(this);})();.

Addition:
(foo.bar)The expression returns what is in parentheses, that is, foo.bar. Then foo.bar is called as a function, i.e. foo.bar(). (foo.bar)表达式返回小括号里的，即foo.bar。然后foo.bar作为函数被调用，即foo.bar()。
而赋值表达式是将右侧的即foo对象的barThe assignment expression assigns the value stored in the bar of the foo object on the right side to the left side and returns it.

Answer 4

var foo = {
bar: function () {

alert(this);

}
};
(foo.bar)(); // Reference, OK => foo (2)
Note: (foo.bar)() here is equivalent to the first foo.bar(), What is executed is the bar method in the foo object. This points to the foo object
(foo.bar = foo.bar)(); // global? (3)
Note: First look at foo.bar = foo.bar, foo .bar is function () {alert(this);} assigned to foo.bar, then (foo.bar = foo.bar) is equal to (function(){alert(this)}), then add the outside () is an immediate function:
(function(){alert(this)})(); so what it points to here is the global window.

Answer 5

The assignment expression returns the global =》window
foo.bar is assigned function(){alert(this)} this points to window globally