JavaScript function expression assigned to a variable (pass by reference or value?)

Question

I have a doubt about the following codes. I thought that data-type primitives assigned to a variable in JS were passed by value, while objects were passed by reference. If functions are objects in JS, I thought they should be passed by reference, but the following two codes behave as passed by value:

CODE 1

var x = function hello(){ console.log("hello")};

console.log(x);

var y = x;

console.log(y);

x = function bye(){console.log("bye")};

console.log(x);

console.log(y);

OUTPUT

[Function: hello]
[Function: hello]
[Function: bye]
[Function: hello]

CODE 2

In this case I assigned a method to a global variable and the behavior is the same:

var x = {name:"Max",
        action: function one(){console.log("one")}
}

var y = x.action;

console.log(y);

x.action = function two(){console.log("two")};

console.log(x.action);

console.log(y);

OUTPUT

[Function: one]
[Function: two]
[Function: one]

I truly appreciate any insight and explanation on this.

You are reassigning variables. Not modifying them by-reference. — Niet the Dark Absol, Aug 13 '16 at 21:27
JS does not have pass by reference. Always pass by value, but that value can be a reference. — Oriol, Aug 13 '16 at 21:49
Thank you for your answer Niet. In code 1, how could I modify the function "hello" assigned to variable x by reference, instead of reassigning it? — max fraguas, Aug 13 '16 at 21:55

score 2 · Answer 1 · answered Aug 13 '16 at 21:30

From that regard, JavaScript is pass-by-value.

Assigning variable X to variable Y will place the value of X in Y, even if the value of X is a reference to an object.

You can see the "by reference" nature that you're looking for when you modify a deeper property in an object:

var x = { foo: () => "bar"; }
var y = x;

x.foo = () => "baz";

y.foo(); // "baz", and not "bar".

If I were to set x to { foo: () => "baz" }, y.foo() would still result in "bar", because I didn't modify the original object, I replaced it with another.

To clarify, JavaScript does not have var x = &y; or something similar like in PHP.

Thank you for your answer Madara. I'm aware of the example you posted. Now, how could I modify in code 1, the function hello, by reference? — max fraguas, Aug 13 '16 at 21:57

score 0 · Answer 2 · answered Aug 13 '16 at 21:29

0

In this code y won't be reference of x later, x is re-assigned with a new function. If you modify a property in x, y will still be a reference of x then.

The same happens with every object in JS.

answered Aug 13 '16 at 21:29

score 0 · Answer 3 · answered Aug 13 '16 at 21:29

0

You are simply assigning functions to variables, there's nothing tricky in this code and it's working as it should, feel free to ask something more.

answered Aug 13 '16 at 21:29

Pexa

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1
7

score 0 · Answer 4 · answered Aug 13 '16 at 21:46

Pass by reference/value doesn't matter here as there is no passing involved.

The values that these variable contain is a reference to a function. I think what you're expecting is that y and x are linked somehow after the assignment but the assignment only changes what function y refers to with no further association with x.

First x holds a reference to hello, then y is set to hold a reference to hello, then x is set to contain a reference to bye. So at the end, x contains a reference to bye, and y contains a reference to hello.

This is basically the same as:

var x = "hello";
var y = x;
x = "bye";

console.log(x);
console.log(y);

JavaScript function expression assigned to a variable (pass by reference or value?)

4 Answers4