Node.js REPL automatically declares underscore after a function declaration or function expression?

Question

I found out something weird about node.js today:

$ node
> console.log(_)
ReferenceError: _ is not defined
    at repl:1:13
    at REPLServer.defaultEval (repl.js:130:27)
    at bound (domain.js:257:14)
    at REPLServer.runBound [as eval] (domain.js:270:12)
    at REPLServer.<anonymous> (repl.js:277:12)
    at REPLServer.EventEmitter.emit (events.js:107:17)
    at REPLServer.Interface._onLine (readline.js:202:10)
    at REPLServer.Interface._line (readline.js:531:8)
    at REPLServer.Interface._ttyWrite (readline.js:812:14)
    at ReadStream.onkeypress (readline.js:101:10)
> function foo() {}
undefined
> console.log(_)
undefined
undefined

The same thing happens after I create a function expression:

$ node
> console.log(_)
ReferenceError: _ is not defined
    at repl:1:13
    at REPLServer.defaultEval (repl.js:130:27)
    at bound (domain.js:257:14)
    at REPLServer.runBound [as eval] (domain.js:270:12)
    at REPLServer.<anonymous> (repl.js:277:12)
    at REPLServer.EventEmitter.emit (events.js:107:17)
    at REPLServer.Interface._onLine (readline.js:202:10)
    at REPLServer.Interface._line (readline.js:531:8)
    at REPLServer.Interface._ttyWrite (readline.js:812:14)
    at ReadStream.onkeypress (readline.js:101:10)
> (function () {}())
undefined
> console.log(_)
undefined
undefined

It's pretty cool, and handy for function arguments which you intentionally want to leave as undefined, but why does it happen? I am using node version v0.11.13 on Arch Linux.

<rant>The whole _ thing is staggeringly frustrating when you're trying to use code that expects _ to be the underscore library. The workarounds are hacky and don't really work; I recompiled to get rid of it--I don't use it anyway.</rant> — Dave Newton
– Dave Newton, Commented Sep 6, 2014 at 15:29
@DaveNewton but isn't repl suppose to work ONLY when you use node via command line? Not in a program itself? I'm confused because I came across this line "By executing node without any arguments from the command-line you will be dropped into the REPL" on the REPL docs. — I_Debug_Everything
– I_Debug_Everything, Commented Sep 6, 2014 at 15:39
+1 as well for bringing this feature into notice. I wouldn't have otherwise known or queried about this. — I_Debug_Everything
– I_Debug_Everything, Commented Sep 6, 2014 at 15:40
@I_Debug_Everything I'm not sure what you mean. If you type node you get the repl. But you can load arbitrary code into the repl, or cut-and-paste, etc. If your code includes the _ symbol things break rather badly. — Dave Newton
– Dave Newton, Commented Sep 6, 2014 at 17:36

mscdex · Accepted Answer · 2014-09-06 15:25:22Z

2

It's a feature of the repl: _ returns the result of the last expression.

Of course, in some cases - like the console.log you have - an expression will not return a result. But yeah it's a handy way to get the value of the last expression.

This only happens in the REPL of course - if you type up the same node program and run it from a file, _ will be under your control.

edited Sep 6, 2014 at 15:25

mscdex

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answered Sep 6, 2014 at 15:20

RyanWilcox

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