Question

I have this code:

function boo() {
    this.is_global = "Yes!";
}

When I run the boo();, I have is_global in window object, I mean the is_global goes into global context.
I expected to have is_global in boo function only.

Is this a normal behavior?

Answer 1

this refers to the window. If you want to refer to the function, use:

arguments.callee.is_global = "Yes!";

Now:

window.is_global; //undefined
boo.is_global; //"Yes!"

An alternative way would be to simply refer to boo by its own name when adding properties to it.

function boo() {
    boo.is_global = "Yes!";
}

Note that this modifies the original boo function object. If you wish to modify all instances of boo resulting from using it as a constructor instead, please see the other answers here.