array variable modification node.js

Question

I can't understand why my variable change

if(chat.users[i + 1])
    console.log("1: " + chat.users[i + 1].username);
if(save[i+1])
    console.log("2: " +  save[i + 1].username);

chat.users[i + 1] = save[i];

if(chat.users[i + 1])
    console.log("3: " + chat.users[i + 1].username);
if(save[i+1])
    console.log("4: " + save[i + 1].username);

I have

1: test1
2: test1
3: test
4: test

I can't understand why I don't have

1: test1
2: test1
3: test
4: test1

Thanks

Edit: The all code is there

http://codepaste.net/gi5ghf (line 92)

Ok, I do it and that's works if(temp[0]) temp[1] = temp[0]; else temp[1] = save[i]; if(save[i+1]) temp[0] = save[i + 1]; chat.users[i + 1] = temp[1]; — Ajouve, Dec 11 '12 at 09:27

score 1 · Accepted Answer · edited May 23 '17 at 10:24

Now with your code it's clear! Let's have a look at it

var chat = {
        users: [
            {
                username: "test"
            },
            {
                username: "test1"
            }
        ]
    },
    // creating reference from save to chat.users
    save = chat.users,
    i = 0;
if (chat.users[i + 1]) {
    // should be chat.users[1].username ("test1")
    console.log("1: " + chat.users[i + 1].username); // output "test1"
}
if (save[i + 1]) {
    // should be save[1].username ("test1")
    console.log("2: " + save[i + 1].username); // output "test1"
}

/*
 * creating reference
 * so chat.users[i + 1] is now save[i] ({ username: "test" })
 * and because save is reference of chat.users, save[i + 1] is now also now save[i] ({ username: "test" })
 */
chat.users[i + 1] = save[i];

if (chat.users[i + 1]) {
    // should be chat.users[1].username ("test")
    console.log("3: " + chat.users[i + 1].username); // output "test"
}
if (save[i + 1]) {
    // should be chat.users[0].username ("test")
    console.log("4: " + save[i].username); // output "test"
}

What?

Let me explain it to you again. For example you got this:

var a = [1, 2];

Now you write this:

var b = a;

Maybe you wanted to copy a to b but you ONLY created a reference!

So have a look at this:

console.log(a, b);
//=> [1, 2] [1, 2]
a[0] = 3;

console.log(a, b);
//=> [3, 2] [3, 2]

b[0] = 4;

console.log(a, b);
//=> [4, 2] [4, 2]

So if you change one value of the object or array it'll be changed for the other one too because it's only a reference and they both got the same memory address.

If you really Want to clone/copy the object/array then have a look at this question.

Sorry, I wrong copy my code, I In my code there where i+1 everywhere — Ajouve, Dec 11 '12 at 08:38
Ok, thanks a lot I understant, I have to copy and not create a reference, Thanks for your link I'll have a look — Ajouve, Dec 11 '12 at 11:48

score 0 · Answer 2 · answered Dec 10 '12 at 23:33

save[i] appears to be a user with a username of 'test'. You are assigning this user to chat.users[i + 1]:

chat.users[i + 1] = save[i];

You are then printing out it its username:

if(chat.users[i + 1]) console.log("3: "+chat.users[i + 1].username);

Finally, you are printing out save[i]'s username:

if(save[i+1]) console.log("4: "+save[i].username);

It prints out 'test', because that's what save[i]'s username is.

Perhaps you meant that last line to print out save[i + 1]'s username, not save[i]'s username? save[i + 1] does have a username of 'test1'.

Sorry, I wrong copy my code, I In my code there where i+1 everywhere — Ajouve, Dec 11 '12 at 08:37

array variable modification node.js

2 Answers2