How to set javascript private variables in constru

Question

Say I have a javascript function/class called Foo and it has a property called bar. I want the value of bar to be supplied when the class is instantiated, e.g:

var myFoo = new Foo(5);

would set myFoo.bar to 5.

If I make bar a public variable, then this works, e.g:

function Foo(bar)
{
    this.bar = bar;
}

But if I want to make it private, e.g:

function Foo(bar)
{
   var bar;
}

Then how would I set the value of the private variable bar such that its available to all internal functions of foo?

Answer 1

One of the best tutorials on private and protected access in javascript is here: http://javascript.crockford.com/private.html.

function Foo(a) {
    var bar = a;                              // private instance data

    this.getBar = function() {return(bar);}   // methods with access to private variable
    this.setBar = function(a) {bar = a;}
}

var x = new Foo(3);
var y = x.getBar();   // 3
x.setBar(12);
var z = x.bar;        // not allowed (x has no public property named "bar")

Answer 2

One way I can think of is to use a closure that's assigned to a name and returns a new object. You would pass in any arguments to the constructor through the call to the closure. That would end up being something like the following:

var fooFactory = function (a, b) {
    var c = 5,
        d = 6,
        foo;

    foo = function (a, b) {
        this.a = a;
        this.b = b;
        this.bar();
    }

    foo.prototype.bar = function () {
        //do something with c and d
        this.c = c + d;
    }

    foo.prototype.getC = function () {
        return c;
    }

    foo.prototype.getD = function () {
        return d;
    }

    return new foo(a, b);
};

This way, a and b are always declared uniquely. You would then construct your object like so:

var obj = fooFactory(1, 2);
//obj contains new object: { a: 1, b: 2, c: 11 }

console.log(obj.getC());
//returns 5

Answer 3

If you are willing to use ES2015 classes,

with ESNext, you can use Javascript private variables like this:

class Foo {
  #bar = '';
  constructor(val){
      this.#bar = val;
  }
  otherFn(){
      console.log(this.#bar);
  }
}

Private field #bar is not accessible outside Foo class.

Answer 4

function Foo(b)
{
   var bar = b;

   this.setBar = function(x){
        bar = x;
   }

   this.alertBar = function(){
        alert(bar);
   }
}

var test = new Foo(10);
alert(test.bar); // Alerts undefined
test.alertBar(); // Alerts 10

Answer 5

I recently had a similar issue but wanted to use accessor properties also. Below is a Foo(Bar) example based on what I came up with for a solution. This example is trivial but can easily be expanded upon using more complex get/set functions.

function Foo(Bar){
    Object.defineProperty(this,"bar",{get:function(){return Bar},set:function(val){Bar=val}});

}
x=new Foo(3);
y=x.bar; //3
x.bar++; //x.bar==4

Answer 6

You have to put all functions that need to access the private variable inside the constructor:

function Foo(bar)
{
  //bar is inside a closure now, only these functions can access it
  this.setBar = function() {bar = 5;}
  this.getBar = function() {return bar;}
  //Other functions
}

var myFoo = new Foo(5);
myFoo.bar;      //Undefined, cannot access variable closure
myFoo.getBar(); //Works, returns 5