const a = {
	x: 0
};
const f = function(a) {
	a.x = 100;
}
f(a);
console.log(a);
// will print { x: 100 }

// =================================

const a = {
	x:0
};
const f = function(a) {
	a = 'foo';
}
f(a);
console.log(a);
// will print { x: 0 }


You're talking about calling a function "by reference". JavaScript (like most other functional languages) doesn't support that, as, changing a variable that is outside the current scope is a side-effect and those contradict functional programming paradigms.
You can always change variables inside the current scope though. So any function defined within another function can change any local variable of the outer one.