为什么将 function 参数作为字符串或引用传递时，“this”会发生变化？

Question

Have a look at this:

var a = {
    b: function() {
      console.log(this); 
    }  
}

// Example 1
a.b(); // a

// Example 2    
eval('a.b()'); // a

// Example 3
setTimeout('a.b()', 100); // a

// Example 4
setTimeout(a.b, 100); // Window

// Example 5
var c = a.b;
c(); // Window

jsFiddle .

Assuming the expected result is what I expected...

Example 1

When calling b() , the property of an Object , this becomes the property's Object , here it's the parent a . It produces the expected result.

Example 2

eval() is meant to adopt its execution context of where it is called, in this case, window . It also produces the expected result.

Example 3

When passing a string to setTimeout() , I'd imagine it is ran through something very similar to eval() . It too produces the expected result.

Example 4

this becomes Window in this example. This is what I am interested in.

Example 5

Here the this becomes Window , because c 's parent object is Window .

1. When passing only a reference to a function (eg ab ), will its this always be Window when called with () ?

2. Is the only way to keep its this as a to pass it as a string to setTimeout() / setInterval() ?

Answer 1

When passing only a reference to a function (egab), will its this always be Window when called with ()?

Yes

Is the only way to keep its this as a to pass it as a string to setTimeout() / setInterval()?

No. Create a new function instead.

setTimeout(function() { a.b() }, 100);

Answer 2

Developers are often confused about javascript's this keyword. The most important thing to remember is that it is provided by the call.

In the 4th example:

 // Example 4 setTimeout(ab, 100); // Window

the first argument is a reference to the function, so it's called without any "parent" object. Since the call doesn't provide an object, this is set to window .

Your comment on example 5:

Here the this becomes Window, because c's parent object is Window.

is not really incorrect. Because the function call doesn't provide an object to use as this , it is set to window (which is the default when no object is provided).

Is the only way to keep its this as a to pass it as a string to setTimeout() / setInterval()?

No. Other than calling it as a property of an object, you can use call or apply :

var x = a.b;
x.call(a);