Pass primitive type to function

In #Javascript , it always pass a copy of value to the function. For example, we pass a variable one, which is Number type, to function changeNumberToTen. It copy the value of one variable, 10, then pass to the function.

function changeNumberToTen(num) {
  num = 10;
}

var one = 1;
console.log(one);  // 1

changeNumberToTen(one);
console.log(one);  // 1

In memory, it may look like below.

The one variable’s reference is different from the function argument num. Though num change the value, it doesn’t change the one’s value.

Pass object to function —property change

What if we pass a object to a function and change it? We pass objA to the function changeObject and the function change the property a of object. Doesn’t it pass by value? Why does objA change?

function changeObject(obj) {
  obj.a = 10;
}

var objA = { a: 5 };

console.log(objA);  //  {a:5}

changeObject(objA);
console.log(objA);  //  {a: 10}

In memory, it may look like below.

It’s still pass by value. The objA’s value is the object’s address and obj’s value is copy of object’s address. Both point to the same object. Therefore, when function changeObject change the property a of object, it affects the objA.

Pass object to function — fully change

What if we pass a object to the function and fully change it? We pass objA to the function changeObject and the function fully change the object to Object({ b: 10 }). Why does objA un-change?

function changeObject(obj) {
  obj = { b: 10 };
}

var objA = { a: 5 };

console.log(objA);  //  {a:5}

changeObject(objA);
console.log(objA);  //  {a:5}

In memory, it may look like below.

Actually, it’s not a matter of pass by value here. So, why the objA doesn’t change? It’s because of reassignment. When we assign new value to a variable, it doesn’t change the existed value in variable but change the variable’s reference to new value. In the example, obj change its reference to the new value (Object({b: 10})’ s address) That’s why objA does not be affected.

Array is also a object in Javascript, so it’ll perform the same result as object does.

Conclusion

In Javascript,

1. Always #pass_by_value . (Someone call it pass by sharing when passing Object)
2. #Assigning new value to variable is #reassignment.

Reference

1. Javascript by reference vs. by value [duplicate] — nrabinowitz@StackOverflow
2. ECMA-262–3 in detail. Chapter 8. Evaluation strategy. — Dmitry Soshnikov
3. Javascript — variables, scope, and memory —  Farhan Faruque@SlideShare