Super Tricky Javascript quiz, need to figure out about the answer

Question

 //1st question var x = 4, obj = { x: 3, bar: function() { var x = 2; setTimeout(function() { var x = 1; alert(this.x); }, 1000); } }; obj.bar(); //2nd question function foo(a) { arguments[0] = 2; alert(a); } foo(1);

1.why it returns 4 instead of 1? i thought this.x refer to 1, but it seems wrong....i just dont understand why it returns 4

2.why it return alert 2 instead of 1, i thought i pass a to function a, and as far as i know, i pass 1 to function foo, and 1 should be alerted because of a(which is 1 when i pass)....i just don't understand why it alert 2

console.log this and you will see that it is referring to the window. on that level x has been defined as 4. This is all about scoping. check out this article, especially the part about this inside of closures ;) javascriptissexy.com/… — HolyMoly
– HolyMoly, Commented May 2, 2016 at 19:37

Pointy · Accepted Answer · 2016-05-02 19:06:58Z

The runtime (in non-strict mode) invokes the setTimeout() callback with this bound to window (the global context), so this.x refers to the outer x.
The arguments object serves as a way to alias the formal parameters of a function. Setting the value of arguments[0] also sets the value of the first declared parameter to the function.

Dmitri Pavlutin · Accepted Answer · 2016-05-02 19:13:24Z

1. why it returns 4 instead of 1?

Notice the first initialization: var x = 4, which in non-strict mode attaches a property x to global object: window.x = 4.

setTimeout(function() { var x = 1; alert(this.x); }, 1000);

setTimout() callback has this context as the global object. And actually calling alert(this.x) -> alert(window.x) -> alert(4).

2.why it return alert 2 instead of 1

arguments object represents the list of function arguments. When modifying it, you actually modify the arguments values: arguments[0] = 2 modifies first argument a = 2.

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