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?
世界只因有你2017-05-19 10:24:14
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.
世界只因有你2017-05-19 10:24:14
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).
大家讲道理2017-05-19 10:24:14
Judging from the code comments, the questioner may just not understand (3). Here is the explanation: foo.bar = foo.bar
The 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bar
The assignment expression assigns the value stored in the bar
of the foo
object on the right side to the left side and returns it.
曾经蜡笔没有小新2017-05-19 10:24:14
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.
習慣沉默2017-05-19 10:24:14
The assignment expression returns the global =》window
foo.bar is assigned function(){alert(this)} this points to window globally