Is the below Javascript function being passed arguments by value or by reference? I'm not sure how to distinguish between the two

Question

I can't figure out what passing by reference and passing by value looks like (I know what they mean in theory). Can you tell me whether the below function is passing arguments by value ore reference? BTW this is my first post :)

var number_1 = 32;
var number_2 = 43;

function addition(num1,num2) {
    alert (num1*num2) ;
}

addition(number_1,number_2); /*is this passing by reference?*/

Answer 1

Primitive types

In javascript number, strings, boolean fall under the category of primitives. And whenever such types are passed as arguments to a function, a separate copy is created inside the function scope which has no impact on the outer scope

for e.g

var number_1 = 32;
var number_2 = 43;
addition(32,43);
//number_1 is 32 here
//number_2 is 43 here

function addition(number_1,number_2) {
    number_1 += 1; // number_1 is 33
    number_2 += 1; // number_2 is 44
}

Reference types

Reference types are slightly different

Take the following example

var obj = new Object();
fn(obj);

function fn(object){
  object.property = "test";
  //obj.property is "test"
  object = new Object();
  object.property = "test 2";
  //obj.property is still "test"
  //obj.property should have changed to "test 2", 
  //if it had been passed by reference
}

if it had been passed by reference , obj.property should have got changed to "test 2" after the last statement inside fn, but it didnt. so when passing reference values to functions, a separate copy of the pointer to the object is passed.

Answer 2

In javascript every variable is passed by value not reference but there is an exception for objects. objects are references (like it was answered here).

which means a number, a string, a boolean or a function don't change at all after you put'em as argument in a function. but an the propertues and the elements inside an object or an array change if they are changed inside a function.

Here is an example

---

Simple variable

function nullify(a) 
{
   a = null;
}

var myVariable = 123;
nullify(myVariable); // myVariable still equals to 123

---

Object

function changeProperty (obj)
{
   obj.key = null;
}

var myObject = new Object();
myObject.key = "value";
changeProperty(myObject); // the property key is null after calling the function

---

Array

function changeArray (Arr)
{
   Arr[0] = null;
}

var myArray = [1,2,3];
changeArray(myArray); // now the value of myArray is [null,2,3]