Question

I have this setter, but I don’t know why set value and it doesn’t change :

    this.setHead = function(head){
        console.log('Head: x:'+this.getHead().getX()+' y:'+this.getHead().getY());
        console.log('temp Head: x:'+head.getX()+' y:'+head.getY());
        this.head = head;
        console.log('Head: x:'+this.getHead().getX()+' y:'+this.getHead().getY());
    }

And the result in Chrome log is :

Head: x:5 y:10 // old value
temp Head: x:1 y:7 //temporary value decide to copy
Head: x:5 y:10     // and the new valụe : NO CHANGE

I have read that Javascript pass object by reference, I don’t know does it difference with Java. If not, I don’t know why happen that. Please tell me.

Thanks 🙂

@ Edited : I have add a line for log, and see strange result :

console.log('Head: x:'+this.head.getX()+' y:'+this.head.getY());
Head: x:1 y:7 

It’s strange, because I think it should be same with below line but It don’t

console.log(‘Head: x:’+this.getHead().getX()+’
y:’+this.getHead().getY());

And my getHead() is :

this.getHead = function() {
            return head;
}

Answer 1

Javascript doesn’t pass anything by reference, setting this.head doesn’t magically make head refer to something else (which is what by reference implies)

Your .getHead() method returns head, not this.head, so the assignment doesn’t affect getHead() at all. They refer to different objects.

Try this:

this.getHead = function() {
    return this.head;
}

---

Basically what you have is most likely:

function Ctor( head ) {

    this.getHead = function() {
        return head;
    }

    this.setHead = function( head ) {
        this.head = head;
    }

}

the setHead sets object property, where as getHead returns the head variable passed to the constructor when initialized.

To avoid this confusion you should just stick to object properties and prototypes and it’s all very simple:

function Ctor( head ) {
    this.head = head;
}

Ctor.prototype.getHead = function() {
    return this.head;
};

Ctor.prototype.setHead = function(head) {
    this.head = head;
};