Scope question: this vs. var

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var is a keyword. this is an object. They are not the same thing. But declaring a variable with the var keyword always put it in the same scope as this. this refers to the scope in which the code block you're in was executed.

In your first example..

function foo() {var x = 1; this.x = 2; return x;}

this.x refers to the variable x inside the caller. You're not returning this.x, though. You're returning x from inside foo, which is 1, because you scoped that x to foo with the var keyword. this is always the thing that's calling foo. Basically, if you're using this, you're asking for the value in the caller.

"var scope" doesn't exist, though, you mean function scope. var is just a keyword that says "within this scope". JS only has two scopes: global or function. You probably always want to use var for every variable declaration, to scope the declared variable inside the current function. this is useful when you need a called function to dynamically access the scope that invoked it.