This document discusses some of the complex and unexpected behaviors in JavaScript. It provides examples of operations with numbers, strings, arrays, and objects that produce unintuitive or surprising results. The examples demonstrate that JavaScript has a very flexible type system that can lead to unintended consequences if not properly understood. The document concludes by providing some links to resources for learning more about JavaScript.

2. About duck typing in dynamic
languages:
“My god how you can write a real program
when you just assigning random crap to
another crap and expecting it to work”
– John
Carmack, founder of id Software, developer of
popular game titles like Wolfenstein , Doom, Quake
and Rage, well-known open source
advocate, millionaire.

3. JavaScript is really
complicated language…

4. var a = 012
console.log( a )
→ 10
OMG!

5. console.log( "3" + 1 )
→ "31"
console.log( "3" – 1 )
→2
OMG!!
console.log( "222" - -"111" )
→ "333"
OMG!!!

6. console.log( 9999999999999999 )
→ 10000000000000000
OMG!!!!
console.log( 111111111111111111111 )
→ 111111111111111110000

7. console.log( Math.max() )
→ -Infinity
console.log( Math.min() )
→ Infinity
console.log( new Array([], null, undefined, null) == ",,,"; )
→ true
var foo = [0];
console.log(foo == !foo);
console.log(foo == foo);
→ true
→ true

8. console.log( 3.toString() )
console.log( 3..toString() )
→ SyntaxError: identifier starts immediately after numeric literal
→ "3"
var a = new Number;
console.log(a == 0)
var a = new String;
console.log(a == "")
var a = new Object;
console.log(a == {})
var a = new Array;
console.log(a == [])
→ true
→ true
→ false
→ false

9. var a = {};
console.log( a == {} );
var a = [];
console.log( a == [] );
→ false
→ false
console.log( (!+[]+[]+![]) );
→ "truefalse"
console.log(
(![]+[])[+[]]+(![]+[])[+!+[]]+([![]]+[][[]])[+!+[]+[+[]]]+(![]+
[])[!+[]+!+[]] )
→ "fail"

10. console.log( {} + {} );
→ NaN
console.log( [] + [] );
→ ""
console.log( [] + {} );
→ "[object Object]"
console.log( {} + [] );
→0
console.log( 1 < 2 < 3 );
console.log( 3 < 2 < 1);
→ true
→ true

11. JavaScript. Just be smarter!

12. Links
WTF JavaScript: http://www.wtfjs.com/
Destroy All Software Screencasts:
http://www.destroyallsoftware.com/
JavaScript Tutorial: http://www.javascript.info/
JavaScript Specification:
http://www.ecma-
international.org/publications/standards/Ecma-
262.htm