CODEWITHSUNDEEP
javascript
Tschallacka
Tschallacka

Reputation: 28722

Storing a pointer in javascript

Is it possible to keep an object reference without using an holder object in javascript? Currently when an object gets overridden I sometimes lose the reference to the "current" object state illustrated in the snippet below;

Is there a way to put a "pointer" in an array or not?

EDIT To the questions asked:

What I have in the objects I have are references to form fields. Some of these are text fields, some of them are textareas, some of them checkboxes.

I wish to keep a map next to the direct referene of what type they are.

basicaly it would be 

obj {
   this.text1 = createTextField();
   this.text1.datepicker();
   this.text2 = createTextField();
   this.area1 = createArea();
   this.check = createCheck();
   this.datefields = [this.text1];
   this.checkboxes = [this.check];
}

So I can use the datefields/checkboxes array as a checkpoint to validate against which type a field is/should behave.

Currently I use 

function datefields() { return [this.text1]; };

But I'd like to know if there's a better way to do this than to intantiate a new array when I need to check it.

I know there is a way with observers to mimic pointer behaviour, and i've fiddled with those and have some good results with that, i'm just curious if there are other ways i'm not aware of.

function myObject() {
    this.myvalue = null;
    this.arr = [this.myvalue];
}

myObject.prototype.alter = function() {
    this.myvalue = "hello";
}

var x = new myObject();
var elem = document.getElementById('results');

function log(message) {
    elem.appendChild(document.createTextNode(message));
    elem.appendChild(document.createElement('br'));
}

log("x.myvalue = "+x.myvalue);
log("x.arr[0] = "+x.arr[0]);
log("calling alter");
x.alter();
log("x.myvalue = "+x.myvalue);
log("x.arr[0] = "+x.arr[0]);   
<div id="results"></div>

Upvotes: 2

Views: 932

Answers (2)

Jason Cust
Jason Cust

Reputation: 10899

Simple answer: Only objects (including all subtypes) are passed by reference in JS. All other simple values are copied.

For a bit more detail I would recommend reading You Don't Know JS: Types & Grammer but specifically the section Value vs Reference in Chapter 2:

In JavaScript, there are no pointers, and references work a bit differently. You cannot have a reference from one JS variable to another variable. That's just not possible.

Quoting further on:

Simple values (aka scalar primitives) are always assigned/passed by value-copy: null, undefined, string, number, boolean, and ES6's symbol.

Compound values -- objects (including arrays, and all boxed object wrappers -- see Chapter 3) and functions -- always create a copy of the reference on assignment or passing.

There are plenty of examples included to show these points. I would highly recommend reading through to get a better understanding of how values/references work in JS.

Upvotes: 1

user1636522
user1636522

Reputation:

There is no pointers in Javascript, though you could cheat a little using a wrapper object. Here is a minimal implementation of such an object:

var Wrapper = function (value) {
    this.value = value;
};

Wrapper.prototype.valueOf = function () {
    return this.value;
};

Then you may use it in place of the original value:

function myObject() {
    this.myvalue = new Wrapper(null); // wrapper
    this.arr = [this.myvalue];
}

myObject.prototype.alter = function() {
    this.myvalue.value = "hello"; // notice the ".value"
}

The rest of your code needs no tweaks.

Upvotes: 0

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