JS WAT

Surprising parts of JavaScript

What to expect from the talk?

Will it be useful?

Maybe a little bit ;)

Why I am here?

- To see some crazy examples of JS behaviour

- To laugh

- To understand the logic behind the scenes

[] == [] // false
[] == ![] // true

Array is not an Array

The abstract equality operator converts both sides to numbers to compare them, and both sides become the number 0 for different reasons.

Explanation

+[] == +![]
0 == +false
0 == 0
true + true

// 2

Math is weird

When the plus operator is placed between two booleans, the booleans are converted to numbers

Explanation

Number(true) + Number(true)
1 + 1
parseInt("f*ck"); // -> NaN

parseInt("f*ck", 16); // -> 15

parseInt is a bad guy

This happens because parseInt will continue parsing character-by-character until it hits a character it doesn't know. The 'f' in 'f*ck' is the hexadecimal digit 15. By default radix is 10 (decimal)

Explanation

999999999999999 // -> 999999999999999

9999999999999999 // -> 10000000000000000
10000000000000000;// -> 10000000000000000
10000000000000000 + 1 // -> 10000000000000000
10000000000000000 + 1.1 // -> 10000000000000002

Magically increasing numbers

This is caused by IEEE 754-2008 standard for Binary Floating-Point Arithmetic. At this scale, it rounds to the nearest even number.

Explanation

1 < 2 < 3 // true
3 > 2 > 1 // false

Comparison knows some jokes

Explanation

1 < 2 < 3 
true < 3 
1 < 3 // true
3 > 2 > 1
true > 1
1 > 1 // false
Math.min(1, 4, 7, 2); // 1
Math.max(1, 4, 7, 2); // 7
Math.min() > Math.max(); // true

Math.max() less than Math.min()

Explanation

Math.min(); // Infinity
Math.max(); // -Infinity

Why is Math.max() less than Math.min()?

null == 0; // false
null > 0; // false
null >= 0; // true

Comparing null to 0

Explanation

null == 0 // false

If you have null on one side of the equal sign, the other side must be null or undefined for the expression to return true. Since this is not the case, false is returned.

null > 0 // false

The algorithm here, unlike that of the abstract equality operator, will convert null to a number.

0 > 0 // false

The >= operator in fact works in a very different way, which is basically to take the opposite of the < operator. 

null >= 0
!(null < 0)
!(0 < 0)
!(false)
[] + {} // "[object Object]"
{} + [] // 0

Funny math

From transposition of elements a sum does not change

Or does it?

Explanation

{} + []
+[] // 0
[] + {}
[].toString() + ({}).toString()
'' + "[object Object]" // "[object Object]"

{} parsed as empty block expresion

1 + {} // "1[object Object]"
{} + 1 // 1

Funny math v2

('b' + 'a' + + 'tman').toUpperCase()
// "BANAN"

Batman is not a Hero he used to be

Explanation

('b' + 'a' + + 'tman').toUpperCase()
('ba' + (+ 'tman')).toUpperCase()
('ba' + NaN).toUpperCase()
('ba' + 'NaN').toUpperCase() // BANAN

Thanks for attention

Resourses and Credits

Contacts

Skype: yavorco
Email: roman.yavoriv@techmagic.co

JS WAT

By Roman Yavoriv

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JS WAT

Surprising parts of JavaScript

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