JavaScript can do that?

You might be wondering why I am writing less these days. I assure you, it's not because I am getting lazy (I am ATM ), it's just that I am on a long overdue holiday. But to keep my juices going, I thought now that I am having fun, let's write a fun post .

In this post I will go through some of the funniest yet unbelievable JavaScript snippets ever. Are you ready?

[] is equal ![]

Array is equal to not array :

;[] == ![] // -> true

What's happening?

The abstract equality operator converts both sides to numbers before comparing them, and both sides will be converted to 0 for different reasons. Arrays are truthy, so the right hand side becomes false which then is coerced to 0. On the left however, the empty array is coerced to a number without becoming a boolean first, and empty arrays are coerced to 0, despite being truthy .

true is false

True is false:

!!'false' == !!'true' // -> true!!'false' === !!'true' // -> true

What's happening?

true is truthy and is represented by value 1 (number), true in string form, is NaN. So:

true == 'true' // -> falsefalse == 'false' // -> false

false is not the empty string, so it's a truthy value, so:

!!'false' // -> true!!'true' // -> true

Cool, ha?

baNaNa

Let's create a banana:

'b' + 'a' + +'a' + 'a' // -> baNaNa

What's happening?

This one is an old trick, remixed. The expression is converted to "ba" + (+"a") + "a", and since "a" is converted to a number, it becomes NaN.

Let's fail

You wouldn't believe this in your wildest dreams, but:

;(![] + [])[+[]] +  (![] + [])[+!+[]] +  ([![]] + [][[]])[+!+[] + [+[]]] +  (![] + [])[!+[] + !+[]]// -> 'fail'

What's happening?

If we break this lot into smaller pieces, we notice that the following pattern occurs often:

![] + [] // -> 'false'![] // -> false

We try adding [] to false, but because of a number of function calls internally, we'll end up converting the right operand into a string:

![] + [].toString() // -> 'false'

Thinking of a string as an array we can access its first character via [0]:

'false'[0] // -> 'f'

The rest is obvious, but the i is tricky. The i in fail is grabbed by generating the string falseundefined and taking the element on index ['10'].

Array equality is evil

Array equality is evil in JavaScript, see below:

[] == ''   // -> true[] == 0    // -> true[''] == '' // -> true[0] == 0   // -> true[0] == ''  // -> false[''] == 0  // -> true[null] == ''      // true[null] == 0       // true[undefined] == '' // true[undefined] == 0  // true[[]] == 0  // true[[]] == '' // true[[[[[[]]]]]] == '' // true[[[[[[]]]]]] == 0  // true[[[[[[ null ]]]]]] == 0  // true[[[[[[ null ]]]]]] == '' // true[[[[[[ undefined ]]]]]] == 0  // true[[[[[[ undefined ]]]]]] == '' // true

What's happening?

The explanation behind this is rather long. So I introduce you to section 7.2.13 Abstract Equality Comparison of the specification.

parseInt is just bad

parseInt is famous by its quirks, I just mention one of the most famous ones:

parseInt('f**k') // -> NaNparseInt('f**k', 16) // -> 15

What's happening?

This happens because parseInt will continue parsing character by character until it hits one it doesn't know. The f in f**k is the hexadecimal digit 15.

NaN is not a number

Type of NaN is a number:

typeof NaN // -> 'number'

What's happening?

Explanations of how typeof and instanceof operators work:

• 12.5.5 The typeof Operator
• 12.10.4 Runtime Semantics: InstanceofOperator(O,C)

Comparison of three numbers

This one is gold:

1 < 2 < 3 // -> true3 > 2 > 1 // -> false

What's happening?

Why does this work that way? Well, the problem is in the first part of an expression. Here's how it works:

1 < 2 < 3 // 1 < 2 -> truetrue < 3 // true -> 11 < 3 // -> true3 > 2 > 1 // 3 > 2 -> truetrue > 1 // true -> 11 > 1 // -> false

You can fix this with greater than or equal operator (>=):

3 > 2 >= 1 // true

Read more about Relational operators in the specification:

• 12.10 Relational Operators

And that's just some of the fun we can have with JavaScript. No wonder some people do not like it, since they don't understand how it works .