JavaScript horrors

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:

[js]

"5" + "10" = "510"

5 + "10" = "510"

"5" + 10 = "510"

5 + 10 = 15

[/js]

[js]

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

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

[/js]

[js]

null + "foo" = "nullfoo"

undefined + "foo" = "undefinedfoo"

null + [1,2] = "null1,2"

undefined + [1,2] = "undefined1,2"

null + null = 0

undefined + undefined = NaN

[/js]

Array constructor:

[js]

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

Array(1, 2, 3) = [1, 2, 3]

Array(4) = [undefined, undefined, undefined, undefined]

[/js]

“Equality”:

[js]

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

[/js]