javascript function scope

Who can explain why result are [20, 20, 10, 10] in this code:

var x = 10;
var foo = {
  x: 20,
  bar: function () {
    var x = 30;
    return this.x;
  }
};

console.log(
  foo.bar(),
  (foo.bar)(),
  (foo.bar = foo.bar)(),
  (foo.bar, foo.bar)()
);

Links to specification are wel开发者_如何学Ccomed

Can't point you at specification, but i can highly recommend reading Douglas Crockford's "Javascript: The good parts". This book will help you understand most of the weird but great features of JavaScript.

As of your question:

1. foo.bar(), this keyword in bar function is bound to foo object
2. (foo.bar)() is the same as above,

3. In javascript you can assign variables from right to left multiple times

   z = 3; x = (y = z); console.log(x); //3

functions are variables as anything else. So you are assigning the function foo.bar to foo.bar, but the parenthesis causes the assigned function to be returned, and then executed.

(foo.bar = foo.bar)(); 
//is the same as
var f = (foo.bar = foo.bar);
f(); 
//and this also the same as:
var f= foo.bar;
f();

The function returned from parenthesis is not bound to anything, so this will refer to global object, in case of browsers - to the window object.

4.. The clause (foo.bar, foo.bar)() is just alike:

a = (3, 4); //last value is returned, first just parsed.
//a contains 4

var f = (foo.bar, foo.bar); 
//f contains body of foo.bar function, 
f() // is executed  in the context of `global` object, eg. `window`. 

Please read about binding of functions in JavaScript.

I think following question will be helpful for this.

The first two function calls are equivalent. They are calling foo's bar method within the context of foo – therefore the value returned by this.x is the value of foo's x property, which is 20.

The second two calls are both questionable/invalid syntax. Try running your code through JSLint and you'll that it spits out several errors and then chokes entirely. My best guess as to why they return 10 is that it's attempting to parse your code in a case where it really shouldn't and is getting confused. 10 might be returned because the browser cannot figure out what you're trying to do and is defaulting to global (window) scope where the value of x is 10. This would also explain Ramesh Vel's comment that the second two values appear as undefined in IE. Due to the fact that the syntax is invalid, different implementations of JavaScript are likely to handle them differently.