Why variable declared without a var in object method does not become a global variable?

Question

Could anyone please explain me..

Why baz which is declared without a var is not becoming a global variable ?

const foo = {
   bar(baz) {

   if (!baz)
   baz = 1;

   }
}

Answer 1

Because it's declared as a function parameter ¹ :

const foo = {
   bar(baz) {
//     ^-------- here

       if (!baz)
           baz = 1;
   }
}

( bar is a function declared using ES2015 method definition syntax.)

If it weren't a parameter, for instance like this:

const foo = {
   bar() {
//     ^----------------- no baz
       if (!baz)
           baz = 1;
   }
}

...then when foo.bar() is called, if baz weren't defined, it would be a ReferenceError (because the Horror of Implicit Globals ² only applies to assigning to undeclared symbols, not reading from them).

---

¹ We frequently call these "arguments" loosely, but "argument" would technically be the actual value passed on a given call; see MDN and Wikipedia for the distinction.

² That's a post on my anemic little blog.

Answer 2

Because it's a parameter.

Including it in the function declaration implicitly declares it for the given function scope.

Answer 3

Because you set it as a function's parameter. In order for a var to become 'global' (which can weaken the resiliency of programs and should be avoided) you should declare it outside of your function.