Global variables and function parameters in JavaScript

Suppose we have a function that takes a number as a parameter:

javascriptfunction func(localNum) {
  console.log(localNum);
}

Let there be the variable num outside the function:

javascriptfunction func(localNum) {
  console.log(localNum);
}

let num = 1; // the external global variable

Let's invoke our function, passing it the variable num as a parameter:

javascriptfunction func(localNum) {
  console.log(localNum);
}

let num = 1;
func(num); // call the function with a parameter, shows 1

It turns out that both the localNum variable (function parameter) and the num variable (as an external variable) will be available inside the function:

javascriptfunction func(localNum) {
  console.log(num); // shows 1
  console.log(localNum); // shows 1
}

let num = 1;
func(num);

The variable localNum itself will be a local variable of the function and will not be accessible from outside:

javascriptfunction func(localNum) {
  
}

let num = 1;
func(num); // call the function with a parameter
console.log(localNum); // will throw an error

Since the variable localNum is local, no changes to it will change anything outside:

javascriptfunction func(localNum) {
  localNum = 2; // it doesn't change anything outside
}

let num = 1;
func(num);

If you change the variable num (global) inside, then the changes will appear outside:

javascriptfunction func(localNum) {
  num = 2;
}

let num = 1;
func(num);
console.log(num); // shows 2

However, if inside the function we declare the variable num through let, then we will thus create the local variable num that does not affect the external variable in any way:

javascriptfunction func(localNum) {
  let num = 2; // declare with let
}

let num = 1;
func(num);
console.log(num); // shows 1 - nothing has changed

javascriptfunction func(localNum) {
  console.log(localNum);
}

func(1);

javascriptfunction func(localNum) {
  console.log(localNum);
}

let num = 1;
func(num);

javascriptfunction func(localNum) {
  console.log(localNum);
}

let num = 1;
func(num);
num = 2;

javascriptlet num = 1;

function func(localNum) {
  console.log(localNum);
}

num = 2;
func(num);

javascriptfunction func(localNum) {
  localNum = 2;
}

let num = 1;
func(num);
console.log(num);

javascriptfunction func(localNum) {
  num = 2;
}

let num = 1;
func(num);
console.log(num);

javascriptfunction func(localNum) {
  let num = 2;
}

let num = 1;
func(num);
console.log(num);