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

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

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?

Solutions/Answers:

Solution 1:

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)
> 

Solution 2:

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';
  }

Solution 3:

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

Related:  get count of items with some property in an array

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.