Inverted parentheses in Javascript

Question

What is it about Javascript that lets me use inverted / backwards parentheses in function calls like this? I'm running in a Node console on the CLI; specifically Node version 0.10.25.

function a(){ return 42 }
a() // -> 42
a)( // -> 42.  WTF?

function b(t){ return t }
b(4) // -> 4
b)4( // No function evaluation; presumably dangling parentheses
b)(4 // -> 4.  WTF?

Addendum: This doesn't appear to work in Chrome 33.0.1750.152, Safari 7.0.2, or Firefox 27.01. Is this actually some kind of "feature" of some interpretation of ECMAScript, or a Node peculiarity? If Node is using V8, shouldn't it match up with the Chrome results?

Perhaps this is browser-specific behavior? I'd think the language would clearly define any errors in this, but perhaps not? (Or perhaps some browsers didn't implement it like that?) — David, Mar 20 '14 at 17:09
Have you tried it in a JS file? I'm guessing this has to do with the REPL, and not JavaScript. — Brad, Mar 20 '14 at 17:10
Why would anyone downvote this question? If you're going to downvote this question you should explain why. There is nothing wrong with this question. — Brad, Mar 20 '14 at 17:10
``I'm running in a Node console on the CLI.`` maybe this is a specific behavior of Node — tetri, Mar 20 '14 at 17:10
Tested with a JS file and it did not work. Interactive did work. Seems like a Node bug to me. — Dark Falcon, Mar 20 '14 at 17:14
I don't know the answer, but I assume the browser is auto-correcting bad syntax to the correct (). — Kevin Florida, Mar 20 '14 at 17:14
I'm betting node's code is evaluated in a pair of `()`, so it becomes `(b)(4)` — cookie monster, Mar 20 '14 at 17:20

score 23 · Accepted Answer · answered Mar 20 '14 at 17:18

23

It is possible that the console wraps everything inside an eval statement: what is actually evaluated is maybe eval(a)(). In that case Chrome returns 42 as well.

answered Mar 20 '14 at 17:18

fabiolune

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Or even just a set of parentheses: `a)(` => `(a)()` – Cᴏʀʏ Mar 20 '14 at 17:20
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Alright now who's going to look inside all the node.js source code to verify that? – Romain Braun Mar 20 '14 at 17:21
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+1 This is definitely the case. If you read here in the docs, you'll see that each line is evaluated using eval: http://nodejs.org/api/repl.html `eval - function that will be used to eval each given line. Defaults to an async wrapper for eval()` – Matthew Blancarte Mar 20 '14 at 17:24
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Guessing this is the line right here: `evalCmd = '(' + evalCmd + ')\n';` https://github.com/joyent/node/blob/master/lib/repl.js#L268 – cookie monster Mar 20 '14 at 17:28
I'm surprised the eval doesn't consider the line as a string. In that case, `eval("a)(")` results in the same trailing-parens non-evaluation. – Bryce Mar 20 '14 at 17:28
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Based on what @cookiemonster found, it's evaluating as `eval('(a)()')` – Matthew Blancarte Mar 20 '14 at 17:33
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@MatthewBlancarte: The cause isn't so much that it's in an `eval` call, but more specifically that the code is wrapped in `"("` and `")"` before being `eval`'d. – cookie monster Mar 20 '14 at 17:33
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@cookiemonster For sure. I was just commenting that it was, indeed, using `eval` Good find there in `repl.js` which totally clears up what's going on. – Matthew Blancarte Mar 20 '14 at 17:36
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@MatthewBlancarte: Sure, just that the answer isn't quite right the way it reads. I'm not even sure that it's actually `eval` that's used, otherwise the code would be able to access the internal local variables. Probably uses the `Function` constructor, or something else. Or maybe it sends it to a safe scope before evaluation. But that's beside the point. We can think of it as `eval` either way. – cookie monster Mar 20 '14 at 17:40
Likewise, defining function a () { return function () { return 2; } } causes 'a)()(a' to return 2 – waTeim Mar 20 '14 at 18:13

Inverted parentheses in Javascript

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