Shattered Crystalline Matrix

Okay, so you probably already know JavaScript is bad; but did you know exactly how bad? Here’s a couple of my favourite examples; if you haven’t seen all of these already, then hopefully you’ll also be saved some future headaches by reading this. Please note that I am using = to denote the concept of structural equality, since there isn’t really any JavaScript operator that maps to that concept.

String coercion:

"5" + "10" = "510"
5 + "10" = "510"
"5" + 10 = "510"
5 + 10 = 15

[1,2,3] + 4 = "1,2,34"
1 + [2,3,4] = "12,3,4"

null + "foo" = "nullfoo"
undefined + "foo" = "undefinedfoo"
null + [1,2] = "null1,2"
undefined + [1,2] = "undefined1,2"
null + null = 0
undefined + undefined = NaN

Array constructor:

Array("1", "2", "3") = ["1", "2", "3"]
Array(1, 2, 3) = [1, 2, 3]
Array(4) = [undefined, undefined, undefined, undefined]

“Equality”:

1 == 1 = true
1 == 2 = false
1 == "1" = true
[1] == 1 = true
[1,2] == "1,2" = true
[1,2,3] == [1,2,3] = false
var x = [1,2,3]; x == x = true
null == undefined = true
null == "null" = false

Okay, so you probably already know JavaScript is bad; but did you know exactly how bad? Here's a couple of my favourite examples; if you haven't seen all of these already, then hopefully you'll also be saved some future headaches by reading this. Please note that I am using = to denote the concept ...