Is JavaScript a pass-by-reference or pass-by-value language?

03/27/2020 01:00:02

The primitive types (number, string, etc.) are passed by value, but objects are unknown, because they can be both passed-by-value (in case we consider that a variable holding an object is in fact a reference to the object) and passed-by-reference (when we consider that the variable to the object holds the object itself).

Although it doesn't really matter at the end, I want to know what is the correct way to present the arguments passing conventions. Is there an excerpt from JavaScript specification, which defines what should be the semantics regarding this?

Answer #1 (1569 Votes)

09/03/2010 22:17:22

It's interesting in JavaScript. Consider this example:

function changeStuff(a, b, c)
{
  a = a * 10;
  b.item = "changed";
  c = {item: "changed"};
}

var num = 10;
var obj1 = {item: "unchanged"};
var obj2 = {item: "unchanged"};

changeStuff(num, obj1, obj2);

console.log(num);
console.log(obj1.item);
console.log(obj2.item);

This produces the output:

10
changed
unchanged
  • If obj1 was not a reference at all, then changing obj1.item would have no effect on the obj1 outside of the function.
  • If the argument was a proper reference, then everything would have changed. num would be 100, and obj2.item would read "changed".

Instead, the situation is that the item passed in is passed by value. But the item that is passed by value is itself a reference. Technically, this is called call-by-sharing.

In practical terms, this means that if you change the parameter itself (as with num and obj2), that won't affect the item that was fed into the parameter. But if you change the INTERNALS of the parameter, that will propagate back up (as with obj1).

1569

Answer #2 (462 Votes)

03/15/2011 21:38:23

It's always pass by value, but for objects the value of the variable is a reference. Because of this, when you pass an object and change its members, those changes persist outside of the function. This makes it look like pass by reference. But if you actually change the value of the object variable you will see that the change does not persist, proving it's really pass by value.

Example:

function changeObject(x) {
  x = { member: "bar" };
  console.log("in changeObject: " + x.member);
}

function changeMember(x) {
  x.member = "bar";
  console.log("in changeMember: " + x.member);
}

var x = { member: "foo" };

console.log("before changeObject: " + x.member);
changeObject(x);
console.log("after changeObject: " + x.member); /* change did not persist */

console.log("before changeMember: " + x.member);
changeMember(x);
console.log("after changeMember: " + x.member); /* change persists */

Output:

before changeObject: foo
in changeObject: bar
after changeObject: foo

before changeMember: foo
in changeMember: bar
after changeMember: bar
462
2
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