Duplicating Javascript object containing private member

Question

Duplicating Javascript object containing private member

I've searched high and low without finding a solution. Why is it so difficult to clone a javascript object with private members without causing them to become quantum entangled?

Take a look at this code... It's a simple private property with getter and setter functions. For some reason, when I call the public setter on one instance, the cloned object also gets changed. How is this happening? Is there a way to fix it?

obj = function(){
    var changed = 0;
    this.getChanged = function(){
        return changed;
    }
    this.setChanged = function(){
        changed = 1;
    }
    this.setUnchanged = function(){
        changed = 0;
    }
};

myObj = new obj();
copiedObj = $.extend(true, {}, myObj); // Or any other deep copy function you'd have around

myObj.setChanged();
myObj.getChanged(); // returns 1
copiedObj.getChanged(); // returns 1!
copiedObj.setUnchanged();
copiedObj.getChanged(); // returns 0
myObj.getChanged(); // returns 0

Any suggestions would be greatly appreciated.

Edit: So far, no luck. I understand that javascript doesn't follow traditional Object Oriented principles like Java or C++, but there must be a workaround. Even if it's not pretty, there has to be a way.

I understand. Solution A: change this.changed instead of var changed
Solution B: create my own cloning function that recreates the entire object from scratch

I was hoping for a Solution C that could make javascript behave more like standard Object Oriented languages.

Can anyone help me out with something other than A or B?

javascript object clone private

Answer 1

Answer №1

The issue at hand is that the variable changed is not private in JavaScript, as it does not support private variables by default. Instead, it is a local variable within the function assigned to the obj variable. By creating functions for getChanged, setChanged, and setUnchanged properties, you are essentially creating closures that capture the variable changed.

When you clone myObj, you are simply creating more references to those functions. This means that whether you access them through myObj or copiedObj, they still refer to the same functions with the same changed variable via closures. Since functions cannot be copied, it might be better to avoid trying to make changed private and just set this.changed = 0; on the second line instead.

Answer 2

The issue at hand is that the variable changed is not private in JavaScript, as it does not support private variables by default. Instead, it is a local variable within the function assigned to the obj variable. By creating functions for getChanged, setChanged, and setUnchanged properties, you are essentially creating closures that capture the variable changed.

When you clone myObj, you are simply creating more references to those functions. This means that whether you access them through myObj or copiedObj, they still refer to the same functions with the same changed variable via closures. Since functions cannot be copied, it might be better to avoid trying to make changed private and just set this.changed = 0; on the second line instead.

Answer 3

Answer №2

When functions are used as constructors, the local variables within them are referred to as "private members" in order to make it clear to Java enthusiasts the purpose of their usage. These local variables do not function like properties; instead, they exist within a lexical scope and are specific to the instance rather than the class, bearing in mind that JavaScript does not adhere to traditional class structures.

One effective approach is to implement a clone method that facilitates deep copying of the instance.

Answer 4

When functions are used as constructors, the local variables within them are referred to as "private members" in order to make it clear to Java enthusiasts the purpose of their usage. These local variables do not function like properties; instead, they exist within a lexical scope and are specific to the instance rather than the class, bearing in mind that JavaScript does not adhere to traditional class structures.

One effective approach is to implement a clone method that facilitates deep copying of the instance.

Answer 5

Answer №3

Copying a function will always result in it being referenced rather than duplicated, even with a deep copy. This is because functions are closures, meaning they share the same private members. Perhaps consider implementing a clone() method to address this issue.

Answer 6

Copying a function will always result in it being referenced rather than duplicated, even with a deep copy. This is because functions are closures, meaning they share the same private members. Perhaps consider implementing a clone() method to address this issue.

Answer 7

Answer №4

Found the solution.

A helpful hint from user123 made a difference.

After realizing that the clone/deep-copy function I originally thought was jQuery's actually came from A. Levy's response on a similar thread, I had to make a couple of adjustments to get it to behave as expected. The original author did mention the necessary changes in their explanation, but I overlooked them during my first read-through.
Firstly, I instantiated the copy with the constructor of the original object and
Removed functions from the for-in loop.

Below is the revised code. Hopefully, it proves useful to someone.

function duplicate(obj) {
    // Handling basic types, null, or undefined
    if (null == obj || "object" != typeof obj) return obj;

    // Handling Dates
    if (obj instanceof Date) {
        var copy = new Date();
        copy.setTime(obj.getTime());
        return copy;
    }

    // Handling Arrays
    if (obj instanceof Array) {
        var copy = [];
        for (var i = 0, len = obj.length; i < len; ++i) {
            copy[i] = duplicate(obj[i]);
        }
        return copy;
    }

    // Handling Objects (excluding functions)
    if (obj instanceof Object) {
        var copy = new obj.constructor();
        for (var attr in obj) {
            if (obj.hasOwnProperty(attr) && !(obj[attr] instanceof Function)) copy[attr] = duplicate(obj[attr]);
        }
        return copy;
    }

    throw new Error("Cannot duplicate obj! Its type is not supported.");
}

Answer 8

Found the solution.

A helpful hint from user123 made a difference.

After realizing that the clone/deep-copy function I originally thought was jQuery's actually came from A. Levy's response on a similar thread, I had to make a couple of adjustments to get it to behave as expected. The original author did mention the necessary changes in their explanation, but I overlooked them during my first read-through.
Firstly, I instantiated the copy with the constructor of the original object and
Removed functions from the for-in loop.

Below is the revised code. Hopefully, it proves useful to someone.

function duplicate(obj) {
    // Handling basic types, null, or undefined
    if (null == obj || "object" != typeof obj) return obj;

    // Handling Dates
    if (obj instanceof Date) {
        var copy = new Date();
        copy.setTime(obj.getTime());
        return copy;
    }

    // Handling Arrays
    if (obj instanceof Array) {
        var copy = [];
        for (var i = 0, len = obj.length; i < len; ++i) {
            copy[i] = duplicate(obj[i]);
        }
        return copy;
    }

    // Handling Objects (excluding functions)
    if (obj instanceof Object) {
        var copy = new obj.constructor();
        for (var attr in obj) {
            if (obj.hasOwnProperty(attr) && !(obj[attr] instanceof Function)) copy[attr] = duplicate(obj[attr]);
        }
        return copy;
    }

    throw new Error("Cannot duplicate obj! Its type is not supported.");
}

Duplicating Javascript object containing private member

Answer №1

Answer №2

Answer №3

Answer №4

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