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[Javascript] The prototype attribute
Posted by Erik on Jan 19 2006 @ 16:48 :: 5663 unique visits
What is it
Lots of people have heard about or seen the prototype attribute but only a few know how to use it (if you already know about it, skip to Extending the Function class below).
The prototype attribute is a class attribute which can be used to add functionallity to classes.
Some simple code:
CODE: JAVASCRIPT
// a simle class
function MyClass() {
this.foo = function() {
return 42;
}
}
// 2 instances of the class
var a = new MyClass();
var b = new MyClass();
// add a function to a (instance of MyClass)
a.bar = function() {
return 1337;
}
b.bar(); // will give an error because the function only exists for a
function MyClass() {
this.foo = function() {
return 42;
}
}
// 2 instances of the class
var a = new MyClass();
var b = new MyClass();
// add a function to a (instance of MyClass)
a.bar = function() {
return 1337;
}
b.bar(); // will give an error because the function only exists for a
This will only add functionallity to the instance of a class. Sometimes you want to add functionallity to the class itself.
This is what the prototype attribute is for:
CODE: JAVASCRIPT
// add the bar function to the class
MyClass.prototype.bar = function() {
return 1337;
}
var c = new MyClass();
c.bar(); // this will return 1337
MyClass.prototype.bar = function() {
return 1337;
}
var c = new MyClass();
c.bar(); // this will return 1337
Where to use it
Adding functionallity to your own classes this way may seem a bit strange because you can just add it to your class definition. But you can't do this for buildin classes such as the Array class:
CODE: JAVASCRIPT
// add a find function to the Array class.
Array.prototype.find = function(item) {
for (var i = 0; i < this.length; i++) {
if (this[i] == item) {
return i; // return the index
}
}
return false;
}
var foo = new Array('hi', 'there', 'this', 'is', 'a', 'test');
foo.find('is'); // will return 3
Array.prototype.find = function(item) {
for (var i = 0; i < this.length; i++) {
if (this[i] == item) {
return i; // return the index
}
}
return false;
}
var foo = new Array('hi', 'there', 'this', 'is', 'a', 'test');
foo.find('is'); // will return 3
Extending the Function class
Warning: advanced readers only!
Because in javascript functions are also classes the prototype attribute can be used to do some cool stuff with functions. Check these 2 functions:
CODE: JAVASCRIPT
Function.prototype.before = function(b) {
var a = this;
return function() {
a();
b();
}
};
Function.prototype.after = function(a) {
return a.before(this);
};
var a = this;
return function() {
a();
b();
}
};
Function.prototype.after = function(a) {
return a.before(this);
};
Example usage:
CODE: JAVASCRIPT
function foo() {
alert('foo');
}
function bar() {
alert('bar');
}
foobar = foo.before(bar);
foobar(); // call foo and then bar
foobar.after(bar)(); // call bar then foo and then bar again
alert('foo');
}
function bar() {
alert('bar');
}
foobar = foo.before(bar);
foobar(); // call foo and then bar
foobar.after(bar)(); // call bar then foo and then bar again
This can be used to creat a chain of functions which can sometimes be usefull for events:
CODE: JAVASCRIPT
function FunctionChain() {
this.call = function() { ; } // starting function does nothing
this.add = function(f) {
this.call = (this.call).before(f);
}
}
var c = new FunctionChain();
c.add(foo);
c.add(foo);
c.add(bar);
c.call(); // this will call them in order they are added: foo, foo, bar
this.call = function() { ; } // starting function does nothing
this.add = function(f) {
this.call = (this.call).before(f);
}
}
var c = new FunctionChain();
c.add(foo);
c.add(foo);
c.add(bar);
c.call(); // this will call them in order they are added: foo, foo, bar
This will not work for functions which expect arguments. So lets add a function to the Function class which will allows you to make a closure from a function:
CODE: JAVASCRIPT
Function.prototype.toClosure = function() {
var f = this;
var a = arguments;
return function() {
return f.apply(this, a);
}
}
var f = this;
var a = arguments;
return function() {
return f.apply(this, a);
}
}
Now you can do stuff like this:
CODE: JAVASCRIPT
// use the same foo function as above but defina a new bar function
function bara(a, b) {
alert('bara: '+(a * b);
}
var foobara = bara.toClosure(42, 1337).after(foo);
foobara();
function bara(a, b) {
alert('bara: '+(a * b);
}
var foobara = bara.toClosure(42, 1337).after(foo);
foobara();
Isn't that cool?
24 comments posted so far
Add your own »
2. On Jan 20 2006 @ 15:18 Erik wrote:
I modiefied the toClosure function to allow the function to return something and to fix the scope the function is executed in.You can now do this:
CODE: JAVASCRIPT
function foo(bar) {
alert(bar+' : '+this.href);
return false;
}
var somelink = document.getElementById('somelink');
somelink.onclick = foo.toClosure(42);
alert(bar+' : '+this.href);
return false;
}
var somelink = document.getElementById('somelink');
somelink.onclick = foo.toClosure(42);
Post about an even more improved toClosure
3. On Feb 14 2006 @ 10:58 Matthijs wrote:
nice :)CODE: JAVASCRIPT
HTMLElement = document.createElement("bla").constructor;
HTMLElement.prototype.setColor(s) {
this.style.color = s;
};
HTMLElement.prototype.setColor(s) {
this.style.color = s;
};
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1. On Jan 20 2006 @ 12:04 Erik wrote:
The toClosure function normally doesn't work on alert because alert isn't a normal Function object and doesn't like apply. You will get a uncaught exception: Illegal operation on WrappedNative prototype object.Here is a way to fix this:
alert = function(str) { // the new alert
__alert(str);
}
setTimeout(alert.toClosure('foobar'), 1000);