Why does calling a function in the Node.js REPL with )( work?

Question

Why is it possible to call function in JavaScript like this, tested with node.js:

~$ node
> function hi() { console.log("Hello, World!"); };
undefined
> hi
[Function: hi]
> hi()
Hello, World!
undefined
> hi)( // WTF?
Hello, World!
undefined
>

Why does the last call, hi)(, work? Is it bug in node.js, bug in V8 engine, officially undefined behaviour, or actually valid JavaScript for all interpreters?

Not working with arguments :-) http://i.stack.imgur.com/jyQwu.png — Royi Namir, Oct 11 '13 at 05:29
doesnt seem to do this in webkit, might be a node specific parser bug (http://jsfiddle.net/hxqrW/) — undefined, Oct 11 '13 at 05:31
Reproducible in node v0.10.20 in Ubuntu 13.04, but not when executed from sublime — thefourtheye, Oct 11 '13 at 05:33
a quick test on jsfiddle.net will show you that it is invalid JavaScript. — Christophe, Oct 11 '13 at 05:35
Seems to be a Node REPL bug, putting the two lines in a `.js` will cause syntax error — leesei, Oct 11 '13 at 05:38
@leesei Well, I was hoping this would be more interesting... Oh well, that seems to be the most accurate answer yet, so if you wanna write that as an answer, we can wrap this up with accepted answer. — hyde, Oct 11 '13 at 05:41
Btw, credit where it is due, this came up at irc (FreeNode #nodejs), by @miniml — hyde, Oct 11 '13 at 06:31
@Royi Did you try putting the arguments outside the asylum? =) — rakslice, Oct 11 '13 at 07:24
Perl has something similar for much the same reason: `perl -ne '$x += $_; }{ print $x'`. See [Hidden features of Perl](http://stackoverflow.com/a/162085/41861) — Adrian Pronk, Oct 11 '13 at 09:04
A better tag name would be, say, `[unexpected-behavior]`. However, that would *also* be a meta tag. — Charles, Oct 12 '13 at 17:14

Jonathan Lonowski · Answer 1 · 2013-10-11T06:45:58.967

It's due to how the REPL evaluates the input, which is ultimately as:

(hi)()

The additional parenthesis are added to force it to be an Expression:

  // First we attempt to eval as expression with parens.
  // This catches '{a : 1}' properly.
  self.eval('(' + evalCmd + ')',
      // ...

The intent is to treat {...} as Object literals/initialisers rather than as a block.

var stmt = '{ "foo": "bar" }';
var expr = '(' + stmt + ')';

console.log(eval(expr)); // Object {foo: "bar"}
console.log(eval(stmt)); // SyntaxError: Unexpected token :

And, as leesei mentioned, this has been changed for 0.11.x, which will just wrap { ... } rather than all input:

  if (/^\s*\{/.test(evalCmd) && /\}\s*$/.test(evalCmd)) {
    // It's confusing for `{ a : 1 }` to be interpreted as a block
    // statement rather than an object literal.  So, we first try
    // to wrap it in parentheses, so that it will be interpreted as
    // an expression.
    evalCmd = '(' + evalCmd + ')\n';
  } else {
    // otherwise we just append a \n so that it will be either
    // terminated, or continued onto the next expression if it's an
    // unexpected end of input.
    evalCmd = evalCmd + '\n';
  }

Does that mean that `hi)(arg` will work? That could be abused to write some truly WTF-ridden code ;-) — Doctor Jones, Oct 11 '13 at 11:43
I still don't understand why that would run. Wouldn't it make a syntax error because of the unmatched open paren? — Peter Olson, Oct 12 '13 at 18:08

score 84 · Accepted Answer · edited Oct 11 '13 at 16:43

84

Seems to be a Node REPL bug, putting these two lines in a .js will cause syntax error.

function hi() { console.log("Hello, World!"); }
hi)(

Error:

SyntaxError: Unexpected token )
    at Module._compile (module.js:439:25)
    at Object.Module._extensions..js (module.js:474:10)
    at Module.load (module.js:356:32)
    at Function.Module._load (module.js:312:12)
    at Function.Module.runMain (module.js:497:10)
    at startup (node.js:119:16)
    at node.js:901:3

Issue submitted #6634.

Reproduced on v0.10.20.

v0.11.7 have this fixed.

$ nvm run 0.11.7
Running node v0.11.7
> function hi() { console.log("Hello, World!"); }
undefined
>  hi)(
SyntaxError: Unexpected token )
    at Object.exports.createScript (vm.js:44:10)
    at REPLServer.defaultEval (repl.js:117:23)
    at REPLServer.b [as eval] (domain.js:251:18)
    at Interface.<anonymous> (repl.js:277:12)
    at Interface.EventEmitter.emit (events.js:103:17)
    at Interface._onLine (readline.js:194:10)
    at Interface._line (readline.js:523:8)
    at Interface._ttyWrite (readline.js:798:14)
    at ReadStream.onkeypress (readline.js:98:10)
    at ReadStream.EventEmitter.emit (events.js:106:17)
>

edited Oct 11 '13 at 16:43

orokusaki

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answered Oct 11 '13 at 05:44

leesei

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They actually went ahead and fixed it? Awwww, pity, I'd really like to see it start a culture and become a _feature_ in all languages. How many times have I typed )( instead of () in a hurry... :)) – geomagas Oct 11 '13 at 13:48
18

@geomagas You think `function a)arg1, arg2( } ]arg2 + arg1[ return; {` should be valid syntax? – azz Oct 11 '13 at 15:45
40

No, not really. Actually, that was a joke. – geomagas Oct 11 '13 at 15:49
7

Once upon a time there was a Lisp implementation with a DWIM option that automatically corrected misspellings and other minor errors. http://en.wikipedia.org/wiki/DWIM – Barmar Oct 11 '13 at 16:32
@leesei Worth noting is that you don't need the trailing `;` after a function declaration, unless you're defining the function anonymously and assigning it to a variable (in which case it is **not** a function declaration - it is a statement). I've updated your answer to reflect as much. – orokusaki Oct 11 '13 at 16:42
@orokusaki yes, my linter also told me. I C&P'ed from the OP. – leesei Oct 11 '13 at 16:47
2

@geomagas, well, some have already went ahead and thought about it - `npm` has `install` **and** `isntall`. bet you haven't noticed :) – Eliran Malka Apr 04 '14 at 01:12
1

@EliranMalka well, after that, I bet we can ask ourselves, isn't all already invented? *runs away* – Félix Adriyel Gagnon-Grenier Apr 02 '17 at 16:10

thefourtheye · Answer 3 · 2013-10-11T06:26:18.263

60

There was a bug raised 4 months back, for this issue https://github.com/joyent/node/issues/5698

And the problem was because, REPL encloses the statements with parens. So

foo)(

becomes

(foo)()

Actual explanation can be found here https://github.com/joyent/node/issues/5698#issuecomment-19487718.

edited Oct 11 '13 at 06:26

answered Oct 11 '13 at 06:14

thefourtheye

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And I thought automatic semicolon insertion was bad. – Eric Lippert Oct 11 '13 at 06:28

Why does calling a function in the Node.js REPL with )( work?

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