How is an array reference into an empty string + true a valid character in JavaScript?

Question

I'm not sure what is happening in this line of javascript:

alert( (''+[][[]])[!+[]+!+[]] ); // shows "d"

What I've figured out:

var a = ! + []; // == true
var b = ! + [] + ! + []; // == 2

It seems that the second part is a reference into an array of letters or some sort, but I don't understand how that is coming from

(''+[][[]])

Also:

alert( (''+[][])[2] ); // nothing happens; console says "unexpected token ]"
alert( (''+[[]][])[2] ); // nothing happens; console says "unexpected token ]"
alert( (''+[[]][[]])[2] ); // shows "d"
alert( (""+true)[2] ); // shows "u"

Someone wrote a tool to convert javascript to '[]{}()+!' characters: http://patriciopalladino.com/blog/2012/08/09/non-alphanumeric-javascript.html — BurnsBA, Aug 10 '12 at 17:38

score 4 · Accepted Answer · edited May 23 '17 at 12:34

4

I'll decompose it for you:

  ('' + [][[]])[!+[]+!+[]]
= ('' + undefined)[!+[]+!+[]]  // [][[]] grabs the []th index of an empty array.
= 'undefined'[! + [] + ! + []]
= 'undefined'[(! + []) + (! + [])]
= 'undefined'[true + true]
= 'undefined'[2]
= 'd'

! + [] == true is explained here What's the significant use of unary plus and minus operators?

edited May 23 '17 at 12:34

Community

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answered Jun 17 '12 at 20:37

Blender

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score 2 · Answer 2 · edited May 23 '17 at 10:34

2

Because "" + true is the string "true", and the third character (index 2) is u.

Things like ! + [] work because + can also be a unary operator, see this SO question.

edited May 23 '17 at 10:34

Community

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answered Jun 17 '12 at 20:21

Dave Newton

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and the `d` is the 3rd char of `undefined`. – Michael Berkowski Jun 17 '12 at 20:22

score 0 · Answer 3 · answered Jun 17 '12 at 20:23

0

alert( (""+true)[2] ); // shows "u"

It's returning the 3rd letter of the string "true".

What does this return?

alert( (''+[[]][[]]));

answered Jun 17 '12 at 20:23

duncan

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How is an array reference into an empty string + true a valid character in JavaScript?

3 Answers3