By what logic js works (!+[]+[]+![]).length returns 9? How is that possible?
As I know js has dynamic types cast but very hard to understand whats going on here
console.log((!+[]+[]+![]).length);
Asked
Active
Viewed 1,338 times
3
Isaac
- 12,042
- 16
- 52
- 116
rebilliony
- 177
- 10
-
1[We had enough JSF explain question](https://stackoverflow.com/questions/7202157/why-does-return-the-string-10?rq=1). – user202729 Jun 24 '18 at 13:53
-
You can refer full list here: http://www.jsfuck.com/ – Isaac Jun 24 '18 at 13:53
-
You'd be a lot closer to answering your own question if you tried logging `!+[]+[]+![]` – JLRishe Jun 24 '18 at 13:53
-
...and then logging `!+[]`, then `!+[]+[]` ... – JJJ Jun 24 '18 at 13:54
-
3(some users keep upvoting "fun" questions. Some "good" one are not upvoted. Not a good thing. **Your upvotes have a big effect, please use them carefully**) – user202729 Jun 24 '18 at 13:55
2 Answers
5
First: +[]
+[] is casting the array to a string, then a number. [] becomes "" and then becomes 0. +[123] is +"123", so it's 123 +[1,2] is +"1,2", this is not a number, so it's NaN
!+[] is !0 (not zero), so it's the boolean true
Second: true + []
Convert both to string, so "true" + "" is "true"
Third: ![]
![] is the boolean false, because an object/array is always thruty.
Fourth: "true" + false
Convert boolean false to a string, so "true" + "false" is "truefalse"
"truefalse".length is 9
progysm
- 1,072
- 6
- 7
3
First we have to understand !+[]+[]+![]:
!+[] = !0 = true
true+[] = "true"
![] = false
"true"+false = "truefalse"
so length of !+[]+[]+![] is length of "truefalse" so it's 9
Firemen26
- 1,025
- 12
- 21