By reference or by value~ Basics ~

In programming, there is the phrase "passing a variable". That is the moment when we call a function with a given parameter. In some languages, those parameters are passed by value in others by reference. In JavaScript we have a bit of a hybrid approach.

The primitives, like numbers and strings, are passed by value and some others like object literals and arrays are passed by "copy of a reference". Let's illustrate this with a couple of examples.

var numOfUsers = 24;
function doSomething(num) {
  num += 1;
}

doSomething(numOfUsers);

console.log(numOfUsers); // 24

Even though we define numOfUsers with var, we are passing it by its value. Meaning that inside the function, we receive its value, but we can not modify the original variable.

Here is another example:

const user = { score: 78 }
function mutates(obj) {
  obj.score += 12;
}

mutates(user);
console.log(user.score); // 90

Here user is passed by a copy of a reference. We can mutate its fields, but we can't amend the original definition. The following snippet proves that:

const user = { score: 78 }
function doesntMutate(obj) {
  obj = { score: 120 }
}

doesntMutate(user);
console.log(user.score); // 78

We have to make the note that in some other languages, the obj = { score: 120 } assignment will change the value of the user object.