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JavaScript Patterns 5.3 Private Properties and Methods
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All object members are public in JavaScript.
var myobj = {
myprop : 1,
getProp : function() {
return this.myprop;
}
};
console.log(myobj.myprop);
// `myprop` is publicly accessible
console.log(myobj.getProp());
// getProp() is public too
The same is true when you use constructor functions to create objects.
// all members are still public:
function Gadget() {
this.name = 'iPod';
this.stretch = function() {
return 'iPad';
};
}
var toy = new Gadget();
console.log(toy.name);
// `name` is public
console.log(toy.stretch());
// stretch() is public
Private Members
Implement private members using a closure.
function Gadget() {
// private member
var name = 'iPod';
// public function
this.getName = function() {
return name;
};
}
var toy = new Gadget();
// `name` is undefined, it's private
console.log(toy.name);
// undefined
// public method has access to `name`
console.log(toy.getName());
// "iPod"
Privileged Methods
it’s just a name given to the public methods that have access to the private members (and hence have more privileges).
In the previous example, getName() is a privileged method because it has “special” access to the private property name.
Privacy Failures
• When you’re directly returning a private variable from a privileged method and this variable happens to be an object or array, then outside code can modify the private variable because it’s passed by reference.
function Gadget() {
// private member
var specs = {
screen_width : 320,
screen_height : 480,
color : "white"
};
// public function
this.getSpecs = function() {
return specs;
};
}
var toy = new Gadget(), specs = toy.getSpecs();
specs.color = "black";
specs.price = "free";
console.dir(toy.getSpecs());
/*
|
color |
"black" |
|
price |
"free" |
|
screen_height |
480 |
|
screen_width |
320 |
*/
Solutions
- Principle of Least Authority (POLA):
Return a new object containing only some of the data that could be interesting to the consumer of the object.
- Another approach, when you need to pass all the data, is to create a copy of the specs object, using a general-purpose object-cloning function.
Object Literal and Privacy
var myobj;
// this will be the object ( function() { // private members var name = "my, oh my"; // implement the public part // note -- no `var` myobj = { // privileged method getName : function() { return name; }
}; }()); var myobj = ( function() { // private members var name = "my, oh my"; // implement the public part return { getName : function() { return name; }
}; }()); myobj.getName();
// "my, oh my"
Prototypes and Privacy
One drawback of the private members when used with constructors is that they are recreated every time the constructor is invoked to create a new object. To solve this you can add common properties and methods to the prototype property of the constructor.
function Gadget() {
// private member
var name = 'iPod';
// public function
this.getName = function() {
return name;
};
}
Gadget.prototype = ( function() {
// private member
var browser = "Mobile Webkit";
// public prototype members
return {
getBrowser : function() {
return browser;
}
};
}());
var toy = new Gadget();
console.log(toy.getName());
// privileged "own" method
console.log(toy.getBrowser());
// privileged prototype method
Revealing Private Functions As Public Methods
var myarray;
(function () {
var astr = "[object Array]",
toString = Object.prototype.toString;
// private method
function isArray(a) {
return toString.call(a) === astr;
})
// private method
function indexOf(haystack, needle) {
var i = 0,
max = haystack.length;
for (; i < max; i += 1) {
if (haystack[i] === needle) {
return i;
}
}
return−1;
}
myarray = {
// public methods
isArray: isArray,
indexOf: indexOf,
inArray: indexOf
};
}());
myarray.isArray([1, 2]); // true
myarray.isArray({
0: 1
}); // false
myarray.indexOf(["a", "b", "z"], "z"); //
myarray.inArray(["a", "b", "z"], "z"); //
Now if something unexpected happens, for example, to the public indexOf(), the private indexOf() is still safe and therefore inArray()will continue to work:
myarray.indexOf = null; myarray.inArray(["a", "b", "z"], "z"); //
References :
JavaScript Patterns - by Stoyan Stefanov (O`Reilly)
