Workshop Good Practices in JavaScript. Regular Expressions. Scopes & Closures. Memory Leaks. Presentado por Ing. Mario García

1. Front End Workshops
II. (Not so) Basic JavaScript
concepts
Mario García Martín
mgarcia@visual-engin.com

2. Good practices in JavaScript
“Always code as if the guy who ends up maintaining your code will
be a violent psychopath who knows where you live.”
— John Woods

3. Avoid global variables
// homer.js
var isDrunk = true;
// bart.js
var isDrunk = false;
// index.html
<script src=”./homer.js”></script>
<script src=”./bart.js”></script>
console.log(isDrunk);
Global variables and functions can be overwritten by other
scripts.

4. Declare variables with var
Using ‘use strict’ directive will prevent undeclared variable
definitions.
function defineBeer() {
beer = 1;
}
defineBeer();
Not doing so results in global variable declaration.
'use strict';
function defineBeer() {
beer = 1;
}
defineBeer();
Uncaught ReferenceError: beer is
not defined
function defineBeer() {
var beer = 1;
}
defineBeer();

5. Manipulating the DOM
$('#homer).addClass('alcoholic');
// Some logic here...
$('#homer').addClass('angry');
Writing the DOM
Reading the DOM
var $homer = $('#homer');
$homer.addClass('alcoholic');
// Some logic here...
$homer.addClass('angry');
var $ul = $('#target-ul');
for (var i=0; i<4; i++) {
$ul.append('<li>' + i +
'</li>');
}
var html = '';
for (var i=0; i<4; i++) {
html += '<li>' + i + '</li>';
}
$('#target-ul').append(html);

6. Avoid heavy nesting
function logHomerBehaviour(homer) {
if (homer.isAtWork()) {
for (var i=0; i<4; i++) {
if (homer.isAsleep()) {
if (homer.isSnoring()) {
for (var j=0; j<2; j++) {
snore += 'Zz...';
}
console.log(snore);
}
} else {
console.log('Doughnuts!');
}
}
}
}
function logHomerBehaviour(homer) {
if (!homer.isAtWork()) { return; }
for (var i=0; i<4; i++) {
if (homer.isAsleep()) {
logHomerSleeping(homer);
} else {
console.log('Doughnuts!');
}
}
}
function logHomerSleeping(homer) {
if (!homer.isSnoring()) { return; }
console.log('Zz...Zz...');
}

7. Comment your code
“Good code explains itself”.
function handleEvent(ev) {
// In IE9 and earlier, use the window.event.
ev = ev || window.event;
}
Comment what you consider needed, but don’t tell others
your life story.

8. Comment your code
“Good code explains itself”.
function handleEvent(ev) {
// In IE9 and earlier, use the window.event.
ev = ev || window.event;
}
Comment what you consider needed, but don’t tell others
your life story.

9. Other good practices
Avoid eval function
Never pass a string to setTimeout or setInterval
Use === instead of ==
console.log(0 == false); // true
console.log('2' == 2); // true
console.log(0 === false); // false
console.log('2' === 2); // false
Tools: JS Lint

10. The only valid measurement of code quality...
WTFs/minute

11. More information in...
● http://www.slideshare.net/cheilmann/javascript-best-practices-1041724
● https://www.youtube.com/watch?v=hQVTIJBZook
● https://www.devbridge.com/articles/javascript-best-practices/
● http://www.codeproject.com/Articles/580165/JavaScript-Best-Practices

12. (Ir)Regular expressions
“Some people, when confronted with a problem, think ‘I know, I’ll
use regular expressions.’ Now they have two problems.”
— Jamie Zawinski

13. Creating a regular expression
var regexp1 = new RegExp('abc', 'gi');
var regexp2 = /abc/gi;
/abc/ A sequence of characters
/[abc]/ Any character from a set of characters
/[^abc]/ Any character not in a set of characters
/[0-9]/ Any character in a range of characters
/x+/ One or more occurrences of pattern x
/x*/ Zero or more occurrences
/x?/ Zero or one occurrence
/x{2,4}/ Between two and four occurrences
/(abc)/ A group
/a|b|c/ Any one of several patterns
/d/ Any digit character
/w/ An alphanumeric character [a-zA-Z0-9_]
/s/ Any whitespace character
/./ Any character except newlines
/^/ Start of input
/$/ End of input

14. Using a regular expression in JavaScript
/[0-9]/.test('in 1992'); //true
var neighbor = /neighbou?r/;
neighbor.test('neighbour'); // true
neighbor.test('neighbor'); // true
Through the RexExp object
Through the String object
'12345'.match(/(d)(d)+/); // ['12345', '1', '5']
'Homer drinks beer'.search(/beer/); // 13
'Ho+me[]r'.replace(/[^ws]/g, ''); // 'Homer'
'Homer drinks beer'.split(/s/); // ['Homer', 'drinks', 'beer']
var match = /d+/.exec('one two 100');
console.log(match); // ['100']
console.log(match.index); // 8

15. Do not abuse regular expressions...
^(?:(?:(?:0?[13578]|1[02])(/|-)31)|(?:(?:0?[1,3-9]|1[0-2])(/|-
)(?:29|30)))(/|-)(?:[1-9]ddd|d[1-9]dd|dd[1-9]d|ddd[1-
9])$|^(?:(?:0?[1-9]|1[0-2])(/|-)(?:0?[1-9]|1d|2[0-8]))(/|-)(?:[1-
9]ddd|d[1-9]dd|dd[1-9]d|ddd[1-9])$|^(0?2(/|-)29)(/|-
)(?:(?:0[48]00|[13579][26]00|[2468][048]00)|(?:20)?(?:0[48]|[2468][048]|[13579][
26]))$

16. More information in...
● Mastering Regular Expressions, by Jeffrey E.F. Friedl
● Introducing Regular Expressions, by Michael Fitzgerald
● Regular Expressions Cookbook, by Jan Goyvaerts and Steven Levithan
● https://developer.mozilla.org/en/docs/Web/JavaScript/Guide/Regular_E
xpressions
● http://eloquentjavascript.net/09_regexp.html
● http://www.cheatography.com/davechild/cheat-sheets/regular-
expressions/

17. Scopes and Closures. Prototype
“First learn computer science and all the theory. Next develop a
programming style. Then forget all that and just hack.”
— George Carrette

18. Lexical scope
In JavaScript, scopes are declared by functions, not by
blocks.
// Global scope
if (true) {
var x =
24;
}
console.log(x);
var x;
if (true) {
x = 24;
}
console.log(x);
Hoisting

19. Lexical scope
var hero = aHero();
var newSaga = function() {
var foil = aFoil();
var saga = function() {
var deed = aDeed();
console.log(hero + deed + foil);
}
}

20. Execution context (in-memory scope)
var hero = randStr();
var newSaga = function() {
var foil = randStr();
var saga = function() {
var deed = randStr();
console.log(hero + deed + foil);
}
saga();
saga();
}
newSaga();
newSaga();
hero = ‘gal’
newSaga = [ Function ]
foil = ‘cow’
saga = [ Function ]
deed = ‘eyes’
foil = ‘cow’
saga = [ Function ]

21. Execution context (in-memory scope)
var hero = randStr();
var newSaga = function() {
var foil = randStr();
var saga = function() {
var deed = randStr();
console.log(hero + deed + foil);
}
saga();
saga();
}
newSaga();
newSaga();
hero = ‘gal’
newSaga = [ Function ]
foil = ‘cow’
saga = [ Function ]
foil = ‘cow’
saga = [ Function ]
deed = ‘tips’

22. Execution context (in-memory scope)
var hero = randStr();
var newSaga = function() {
var foil = randStr();
var saga = function() {
var deed = randStr();
console.log(hero + deed + foil);
}
saga();
saga();
}
newSaga();
newSaga();
hero = ‘gal’
newSaga = [ Function ]
foil = ‘cow’
saga = [ Function ]
foil = ‘cat’
saga = [ Function ]
deed = ‘Rubs’

23. Execution context (in-memory scope)
var hero = randStr();
var newSaga = function() {
var foil = randStr();
var saga = function() {
var deed = randStr();
console.log(hero + deed + foil);
}
saga();
saga();
}
newSaga();
newSaga();
hero = ‘gal’
newSaga = [ Function ]
foil = ‘cow’
saga = [ Function ]
foil = ‘cat’
saga = [ Function ]
deed = ‘Robs’

24. ‘this’ keyword
Invocation as a method
var homer = {
beer: 'Nice to have another one!',
info: function() {
console.log(this === homer);
console.log(this.beer);
}
};
homer.info(); // true, 'Nice to have another one!'
var bart = {
beer: 'Too young'
};
bart.info = homer.info;
bart.info(); // false, 'Too young'
function bar() {
console.log(this);
}
bar(); // global

25. ‘this’ keyword
Invocation as a method
var anum = 0;
var foo = {
anum: 10,
baz: {
anum: 20,
bar: function() {
console.log(this.anum);
}
}
}
foo.baz.bar(); // 20
var hello = foo.baz.bar;
hello(); // 0
var foo = {
baz: function() {
console.log(this);
}
}
foo.baz(); // foo
var anotherBaz = foo.baz;
anotherBaz(); // global

26. ‘this’ keyword
Invocation as a constructor
function Person() {
this.x = 0;
}
var person = new Person();
console.log(person.x); // 0

27. ‘this’ keyword
Invocation with the apply and call methods
function juggle() {
var result = 0;
for (var n = 0; n < arguments.length; n++) {
result += arguments[n];
}
this.result = result;
}
var ninja1 = {};
var ninja2 = {};
juggle.apply(ninja1, [1, 2, 3, 4]); // ninja1.result = 10;
juggle.call(ninja2, 5, 6, 7, 8); // ninja2.result = 26;

28. Closures
Combines two things: a function, and the environment in which that function
was created.
var toast = 'Cheers!';
function makeToast() {
console.log(toast);
}
makeToast();
function
makeToast() { ... }
var toast

29. Closures
var scope = 'global scope';
function checkscope() {
var scope = 'local scope';
function f() { return
scope; }
return f();
}
checkscope(); // 'local scope';
var scope = 'global scope';
function checkscope() {
var scope = 'local scope';
function f() { return
scope; }
return f;
}
var func = checkscope();
func(); // 'local scope';

30. Closures
var fns = [];
for (var i=0; i<4; i++) {
fns.push(function() {
console.log(i);
});
}
fns[0](); // 4
fns[1](); // 4
fns[2](); // 4
fns[3](); // 4
var fns = [];
for (var i=0; i<4; i++) {
fns.push( (function(a) {
return function() {
console.log(a);
};
})(i) );
}
fns[0](); // 0
fns[1](); // 1
fns[2](); // 2
fns[3](); // 3

31. Constructor function
function Animal(_name) {
var name = _name;
this.getName = function() {
console.log('My name is ' + name);
};
this.setName = function(_name) {
name = _name;
};
}
var animal = new Animal('Santa');
animal.getName(); // My name is Santa
animal.setName('New Santa');
animal.getName(); // My name is New Santa

32. Prototype chain
var o = { a: 1 };
// o --> Object.prototype --> null
var a = ['Homer', 'Marge', 'Lisa'];
// a --> Array.prototype --> Object.prototype --> null
function f() { return 2; }
// f --> Function.prototype --> Object.prototype --> null

33. Prototype chain
function Animal(_name) {
// Instance properties can be set on each instance of the class
this.name = _name;
}
// Prototype properties are shared across all instances of the class.
Animal.prototype.getName = function() {
console.log('My name is ' + this.name);
};
var animal = new Animal('Santa');
animal.getName(); // My name is Santa

34. Inheritance
function Cat(_name) {
Animal.call(this, _name);
}
Cat.prototype = new Animal();
Cat.prototype.scratch = function() {
console.log('I love to scratch!');
}
var cat = new Cat('Claws');
cat.getName(); // My name is Claws
cat.scratch(); // I love to scratch!

35. More information in...
● Secrets of the JavaScript Ninja, by John Resig
● Javascript, the definitive guide, by David Flanagan
● Scope & closures, by Kyle Simpson
● http://www.sitepoint.com/demystifying-javascript-variable-scope-
hoisting/
● http://davidshariff.com/blog/what-is-the-execution-context-in-
javascript/
● http://davidshariff.com/blog/javascript-scope-chain-and-closures/
● http://davidshariff.com/blog/javascript-this-keyword/

36. Memory leaks
“‘Yeah, it works but you’re leaking memory everywhere. Perhaps
we should fix that”. I’ll just restart apache every 10 requests.”
— Rasmus Lerdorf

37. Circular references
<div id="target">Element</div>
<script type="text/javascript">
var obj = document.getElementById('target');
document.getElementById('target').property = obj;
obj.bigString = new Array(10000).join('*');
</script>
Example 1

38. Circular references
<div id="target"></div>
<script type="text/javascript">
function myFunction(element) {
this.elementReference = element;
element.property = this;
}
function leak() {
new myFunction(document.getElementById('target'));
}
leak();
</script>
Example 2

39. Closures and circular references
<button id="element">Click Me</button>
<script type="text/javascript">
function outerFunction() {
var obj = document.getElementById('element');
obj.onclick = function innerFunction() {
console.log('Hi! I will leak');
};
obj.bigString = new Array(10000).join('*');
};
outerFunction();
</script>

40. Closures. Break the circular reference
<button id="element">Click Me</button>
<script type="text/javascript">
function outerFunction() {
var obj = document.getElementById('element');
obj.onclick = function innerFunction() {
console.log('Hi! I have avoided the leak');
};
obj.bigString = new Array(10000).join('*'));
obj = null; //This breaks the circular reference
};
outerFunction();
</script>

41. Closures. Add another closure
<button id="element">Click Me</button>
<script type="text/javascript">
function outerFunction() {
var anotherObj = function innerFunction() {
console.log('Hi! I have avoided the leak');
};
(function anotherInnerFunction() {
var obj = document.getElementById('element');
obj.onclick = anotherObj;
})();
};
outerFunction();
</script>

42. Closures. Avoid the initial closure
<button id="element">Click Me</button>
<script type="text/javascript">
function outerFunction() {
var obj = document.getElementById('element');
obj.onclick = doesNotLeak;
}
function doesNotLeak() {
console.log('Hi! I have avoided the leak');
}
outerFunction();
</script>

43. More information in...
● https://developer.mozilla.org/en-
US/docs/Web/JavaScript/Memory_Management
● http://www.ibm.com/developerworks/web/library/wa-memleak/
● https://msdn.microsoft.com/en-us/magazine/ff728624.aspx

44. Thanks for your attention!
Leave your questions on the comments section