What would be the explanation for the strange javascript expression?

Question

In JavaScript the following line of code gives answer as 1

+ ! {} [true]

I couldn't understand how?

Any gurus explanation is appreciated.

related: [What is the explanation for these bizarre JavaScript behaviours mentioned in the 'Wat' talk for CodeMash 2012?](http://stackoverflow.com/questions/9032856/what-is-the-explanation-for-these-bizarre-javascript-behaviours-mentioned-in-the) — Bergi, May 10 '13 at 11:32
Without minutes this puzzle was solved. I love you 'SO' community. thanks a lot. — rajakvk, May 10 '13 at 11:34

Romain Braun · Accepted Answer · 2013-05-10T10:31:12.987

4

{} is an empty object.

So {}[0] or {}[true] or {}[1] etc.. are undefined

adding ! casts {}[0] as a boolean, returning the opposite. (undefined becoming false, it therefore returns true).

adding + casts it as an int, so true becomes 1.

edited May 10 '13 at 10:31

answered May 10 '13 at 09:56

Romain Braun

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did the question change from [0] to [true]? – Matthew Gilliard May 10 '13 at 09:59
2

No, `{}` is not undefined. It's an empty object. – georg May 10 '13 at 10:02
@RomainBraun: actually, this is trickier than it seems. I've posted an explanation: http://stackoverflow.com/a/16480480/989121 – georg May 10 '13 at 10:43
plus one for the + casting tip. Needed it just after I read this. – Stevo May 10 '13 at 10:54

rab · Answer 2 · 2013-05-10T12:18:23.840

2

I tried to explain it through code .

var emptyObject = {};
    valueOfUndefinedKey = emptyObject['key_not_exists'],
    itsNot = !valueOfUndefinedKey ,
    finalConvertedNumber = +itsNot ;

console.log(
    emptyObject,
    valueOfUndefinedKey,
    itsNot,
    finalConvertedNumber
)

which prints

Object {}

undefined

true

1

edited May 10 '13 at 12:18

answered May 10 '13 at 10:08

rab

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What would be the explanation for the strange javascript expression?

2 Answers2