Pass by Value and Pass by Reference in Javascript

Video by Arnav Gupta Tutorial by Apoorvaa Gupta

A variable can be passed to the function in two ways. Either by value or by reference.

When we pass a variable by value to a function, a new copy of the variable is created inside the function and all the changes to the variable are made to that copy of the variable.

When we pass a variable by reference to a function, the reference to that same variable is passed to the function so any change in that variable is reflected back in the main function.

In javascript we can pass variables only by value to a function.

function change_fruit(fruit){
    console.log("Original Value: " + fruit);        //Output: Original Value: Apple
    console.log("Modified Value: " + fruit);        //Output: Modified Value: Mango

var f="Apple"
console.log(f);     // Output: Apple
console.log(f);     // Output: Apple

This means that the value of variable f is not changed. The change only took place in the copy of the variable f that is variable fruit.

This is not the case when we pass non-primitive variables to a function. By non-primitives we mean that the variable which are not numbers or strings or booleans. In case of non-primitives the value of the variable is the reference to the non-primitive Object/value. So in this case, passing the value means that the reference to the non-primitve is passed and hence changes are reflected back. This is not a case of pass by reference. The variable is passed by value only.

function change_numbers(arr){

var numbers = [1,2];
console.log(numbers);       // Output: [1, 2]
console.log(numbers);       // Output: [5, 10]

So when we pass array numbers to the function, the reference to [1,2] is passed to the function. Hence the changes are reflected back.