this is a sample introduction about closure, the sample code in this slides is written in javascript

1. Closure in JavaScript
By Nicholas Ren,
ThoughtWorks

2. JavaScript
 The world’s most misunderstood programing language
 looks like an ordinary procedural language.
 has more common with functional language.
 closure, functions are first class.

3. Closure
 What is it?
 How to create a closure?
 What’s its usage?
 Advantage and disadvantage

4. What’s a closure?
 closure is a function together with a referencing environment
for the non-local variables of that function
 closure is a specified kind of object that combines two things:
a function, and the environment in which that function was
created
 a function remembers what happens around it

5. How to create a closure?
 crate a inner function and return it to outside.
 sample:
function makeAdder(x)
{
return function(y)
{
return x + y
}
}
var add5 = makeAdder(5);
var add1000 = makeAdder(1000);
alert("add5(10)): " + add5(10));
alert("add1000(10)): " + add1000(10));

6. Usages of closure
 handle event driven function
 emulate private method

7. Sample:
handle event driven functions
HTML:
<p id="help">Helpful notes will appear here</p>
<p>E-mail: <input type="text" id="email" name="email"></p>
<p>Name: <input type="text" id="name" name="name"></p>
<p>Age: <input type="text" id="age" name="age"></p>

8. Sample:
handle event driven functions
javascript:
function showHelp(help) {
document.getElementById('help').innerHTML = help;
}
function setupHelp() {
var helpText = [
{'id': 'email', 'help': 'Your e-mail address'},
{'id': 'name', 'help': 'Your full name'},
{'id': 'age', 'help': 'Your age (you must be over 16)'}
];
for (var i = 0; i < helpText.length; i++) {
var item = helpText[i];
document.getElementById(item.id).onfocus = function() {
showHelp(item.help);
}
}
}
setupHelp();

9. Sample:
handle event driven functions
javascript:
function showHelp(help) {
document.getElementById('help').innerHTML = help;
}
function makeHelpCallback(help) {
return function() {
showHelp(help);
};
}
function setupHelp() {
var helpText = [
{'id': 'email', 'help': 'Your e-mail address'},
{'id': 'name', 'help': 'Your full name'},
{'id': 'age', 'help': 'Your age (you must be over 16)'}
];
for (var i = 0; i < helpText.length; i++) {
var item = helpText[i];
document.getElementById(item.id).onfocus = makeHelpCallback(item.help);
}
}
setupHelp();

10. Sample:
emulate private function
Person = function(){
name = "Adam";
return {
getName:function getName(){ return name;},
setName:function setName(new_name){
name = new_name;
}
}
}
p = new Person();
p.setName("Jim");
alert(p.getName());

11. Advantage & Disadvantage
 Advantage
 bring any object along with the scope chain
 data encapsulation
 make event handling logic more clear
 Disadvantage:
 slow speed
 more memory consumption

12. Theory and implementation
 Closures are typically implemented with a special data
structure that contains a pointer to the function code, plus a
representation of the function's lexical environment (e.g., the
set of available variables and their values) at the time when
the closure was created.
 difficult to implement for a stack based programing language
because of Funarg problem

13. Related Terms
 function object
 lamda lifting
 first-class function
 high-order function

14. Appendix
 http://javascript.crockford.com/javascript.html
 http://en.wikipedia.org/wiki/Scope_(computer_science)#Lexic
al_scoping
 http://en.wikipedia.org/wiki/Closure_(computer_science)