CODEWITHSUNDEEP
javascriptoopinheritancesingleton
Hasanavi
Hasanavi

Reputation: 8625

Javascript singleton inheritance

I would like to keep a single parent class. all clild classes that inherit the parent class will be able to share the same parent class object. How that can be achieved?

var ParentClass = function(){
    this.a = null;
}

ParentClass.prototype.setA = function(inp){
    this.a = inp;
}

ParentClass.prototype.getA = function(){
    console.log("get a "+this.a);
}

// Clild Class

var ClassB = function(){}

ClassB.prototype = Object.create(ParentClass.prototype);

var b = new ClassB();
b.setA(10);
b.getA(); //it will return 10


//Another clild Class
var ClassC = function(){}

ClassC.prototype = Object.create(ParentClass.prototype);
var c = new ClassC();
c.getA(); //I want 10 here.

I understand, as for the second clild class the parent class is instantiating again that is why I can't access the old object. How I can achieve this singleton inheritance in Javascript? Any idea?

Upvotes: 6

Views: 2007

Answers (2)

Bergi
Bergi

Reputation: 664599

Put such static values somewhere else. this is the current instance, and that's not where you want to create a new property. Choices are:

  • ParentClass.prototype (as demonstrated by @bfavaretto), which will lead to all instances inheriting and being able to overwrite it

  • a scoped variable (implementing the revealing module pattern basically):

    (function() {
    var a;
    ParentClass.prototype.setA = function(inp){
        a = inp;
    };
    ParentClass.prototype.getA = function(){
        console.log("get a "+a);
        return a;
    };
}());

  • the ParentClass function object itself:

    ParentClass.prototype.setA = function(inp){
    ParentClass.a = inp;
};
ParentClass.prototype.getA = function(){
    console.log("get a "+ParentClass.a);
    return ParentClass.a;
};

Upvotes: 3

bfavaretto
bfavaretto

Reputation: 71918

When you call getA from any instance, the value of this inside it will point to the instance itself. You can achieve what you're looking for if your change the setter code to this:

ParentClass.prototype.setA = function(inp){
    ParentClass.prototype.a = inp;
}

Note that calling getA from an instance of ParentClass will return null, and the constructor defines an own property a that will shadow the one from the prototype.

Upvotes: 1

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