What is passed by reference / value in JavaScript?

Question

I am confused as to how JavaScript decides what is passed by value and reference, so I attempted to figure it out by writing some tests. The results have confused me further. Can anyone explain how the interpreter decided if variables were passed by val/ref in this code? Also, do you know how to explicitly pass a variable by reference?

function test(str, obj1, obj2, arr1, arr2) {
    str = "Str was passed by reference!";

    obj1.val = "Obj1 was passed by reference!";
    obj2 = {val: "Obj2 was passed by reference!"};

    arr1[0] = 'r';
    arr1[1] = 'e';
    arr1[2] = 'f';

    arr2 = "arr2 was passed by reference!".split('');
}

var str = "Str was passed by value!";
var obj1 = {val: "obj1 was passed by value!"};
var obj2 = {val: "obj2 was passed by value!"}

var arr1 = ['val'];
var arr2 = "arr2 was passed by value!".split('');

test(str, obj1, obj2, arr1, arr2);
console.log(str);
console.log(obj1);
console.log(obj2);
console.log('arr1 was passed by ' + arr1.join(''));
console.log(arr2.join(''));

This code logs:

Str was passed by value!
Object {val: "Obj1 was passed by reference!"}
Object {val: "obj2 was passed by value!"}
arr1 was passed by ref
arr2 was passed by value!

Thanks :)

I have this code on JSFiddle if you would like to see it yoursef.

--edit--

For context, I originally was trying to explicitly pass in large objects / primitives by reference.

Objects (which includes regular objects, arrays and functions) are passed by reference (or more accurately by pointer since they aren't a true reference). Strings are probably passed by reference too (for efficiency reasons), but since strings are immutable (you can't change a string), there's no difference in behavior if its a reference or a copy. — jfriend00, Feb 13 '15 at 05:51
Why do my 2 objects seem to be inconsistent with that, though? If they were passed in as pointers, why is one object retaining its original value? — , Feb 13 '15 at 05:53
Your `test()` function just assigns a new value to the argument named obj2. It doesn't change the global `obj2` in any way. If you changed any property on that (like you do with `obj1`, you would see the change to that property reflected in the global `obj2`. This is why it's not a true reference because you can't assign a new value back to the original variable through it - but you can change properties on it. — jfriend00, Feb 13 '15 at 05:55
JavaScript does not have pass by reference. Everything is pass by value. That value may be a *reference to an object*. However, objects are **mutable** and hence you see what you see with `obj1` and `arr1`. For a proper explanation of pass by reference see https://en.wikipedia.org/wiki/Evaluation_strategy#Call_by_reference — Felix Kling, Feb 13 '15 at 05:57

Jcl · Answer 1 · 2015-02-13T06:28:51.040

Your testing code is just wrong. In the occations you see you are "passing by reference", that doesn't mean you are "passing by reference", it means you are changing the contents of the object/array, not the object reference itself.

Where you see you are "passing by value", it's because the value you set hasn't changed.

When you assign str, or obj2, or arr2 (the ones your code say are "by value"), you are setting a new reference on the inner scoped parameter variable. You are not changing the outer one. So when you print it (outside the test function) it just maintains the value it was assigned.

Likewise, the ones you see are passed "by reference", you are just changing their properties, the "content" of the variable. If you do:

obj1.val = "Obj1 was passed by reference!";

Then you are setting "val", not "obj1". That's why you see it changes "outside". Or when you do:

arr1[0] = 'r';
arr1[1] = 'e';
arr1[2] = 'f';

You are setting "element 0 of arr1", "element 1 of arr1", etc., not "arr1".

So your code doesn't prove what you are thinking it proves.

You are mutating the objects in some cases, and assigning different objects in others, and that's the actual difference you are seeing, not the "passing by reference or value".

By the way, Javascript can't "pass by reference" in the way you want it to. You can't just set a scoped argument variable value and pretend it changes outside that scope. Other languages allow you to do this, but not Javascript. You pass a reference (a pointer) to the object in the contents of the argument, but the argument itself is not related to the "variable you pass in", in any way (other than they originally point to the same object)

What is passed by reference / value in JavaScript?

1 Answers1