Question

function Foo() {  
    alert(this === a);//return false!   **why this is not equal a?**  
}  
var a = new Foo();//create a new object

foo is a constructor,why this in the ‘Foo’ is not equal with ‘a’.

Answer 1

The assignment to the variable a has not yet happened when you’re inside the constructor. The sequence of events is:

1. Create a new object
2. Run the constructor
3. Assign the new object to the variable a.