Node.js v0.8.24 Manual & Documentation
Table of Contents
REPL#
A Read-Eval-Print-Loop (REPL) is available both as a standalone program and easily includable in other programs. The REPL provides a way to interactively run JavaScript and see the results. It can be used for debugging, testing, or just trying things out.
By executing node
without any arguments from the command-line you will be
dropped into the REPL. It has simplistic emacs line-editing.
mjr:~$ node
Type '.help' for options.
> a = [ 1, 2, 3];
[ 1, 2, 3 ]
> a.forEach(function (v) {
... console.log(v);
... });
1
2
3
For advanced line-editors, start node with the environmental variable
NODE_NO_READLINE=1
. This will start the main and debugger REPL in canonical
terminal settings which will allow you to use with rlwrap
.
For example, you could add this to your bashrc file:
alias node="env NODE_NO_READLINE=1 rlwrap node"
repl.start(options)#
Returns and starts a REPLServer
instance. Accepts an "options" Object that
takes the following values:
prompt
- the prompt andstream
for all I/O. Defaults to>
.input
- the readable stream to listen to. Defaults toprocess.stdin
.output
- the writable stream to write readline data to. Defaults toprocess.stdout
.terminal
- passtrue
if thestream
should be treated like a TTY, and have ANSI/VT100 escape codes written to it. Defaults to checkingisTTY
on theoutput
stream upon instantiation.eval
- function that will be used to eval each given line. Defaults to an async wrapper foreval()
. See below for an example of a customeval
.useColors
- a boolean which specifies whether or not thewriter
function should output colors. If a differentwriter
function is set then this does nothing. Defaults to the repl'sterminal
value.useGlobal
- if set totrue
, then the repl will use theglobal
object, instead of running scripts in a separate context. Defaults tofalse
.ignoreUndefined
- if set totrue
, then the repl will not output the return value of command if it'sundefined
. Defaults tofalse
.writer
- the function to invoke for each command that gets evaluated which returns the formatting (including coloring) to display. Defaults toutil.inspect
.
You can use your own eval
function if it has following signature:
function eval(cmd, context, filename, callback) {
callback(null, result);
}
Multiple REPLs may be started against the same running instance of node. Each will share the same global object but will have unique I/O.
Here is an example that starts a REPL on stdin, a Unix socket, and a TCP socket:
var net = require("net"),
repl = require("repl");
connections = 0;
repl.start({
prompt: "node via stdin> ",
input: process.stdin,
output: process.stdout
});
net.createServer(function (socket) {
connections += 1;
repl.start({
prompt: "node via Unix socket> ",
input: socket,
output: socket
}).on('exit', function() {
socket.end();
})
}).listen("/tmp/node-repl-sock");
net.createServer(function (socket) {
connections += 1;
repl.start({
prompt: "node via TCP socket> ",
input: socket,
output: socket
}).on('exit', function() {
socket.end();
});
}).listen(5001);
Running this program from the command line will start a REPL on stdin. Other
REPL clients may connect through the Unix socket or TCP socket. telnet
is useful
for connecting to TCP sockets, and socat
can be used to connect to both Unix and
TCP sockets.
By starting a REPL from a Unix socket-based server instead of stdin, you can connect to a long-running node process without restarting it.
For an example of running a "full-featured" (terminal
) REPL over
a net.Server
and net.Socket
instance, see: https://gist.github.com/2209310
For an example of running a REPL instance over curl(1)
,
see: https://gist.github.com/2053342
Event: 'exit'#
function () {}
Emitted when the user exits the REPL in any of the defined ways. Namely, typing
.exit
at the repl, pressing Ctrl+C twice to signal SIGINT, or pressing Ctrl+D
to signal "end" on the input
stream.
Example of listening for exit
:
r.on('exit', function () {
console.log('Got "exit" event from repl!');
process.exit();
});
REPL Features#
Inside the REPL, Control+D will exit. Multi-line expressions can be input. Tab completion is supported for both global and local variables.
The special variable _
(underscore) contains the result of the last expression.
> [ "a", "b", "c" ]
[ 'a', 'b', 'c' ]
> _.length
3
> _ += 1
4
The REPL provides access to any variables in the global scope. You can expose
a variable to the REPL explicitly by assigning it to the context
object
associated with each REPLServer
. For example:
// repl_test.js
var repl = require("repl"),
msg = "message";
repl.start("> ").context.m = msg;
Things in the context
object appear as local within the REPL:
mjr:~$ node repl_test.js
> m
'message'
There are a few special REPL commands:
.break
- While inputting a multi-line expression, sometimes you get lost or just don't care about completing it..break
will start over..clear
- Resets thecontext
object to an empty object and clears any multi-line expression..exit
- Close the I/O stream, which will cause the REPL to exit..help
- Show this list of special commands..save
- Save the current REPL session to a file.save ./file/to/save.js
.load
- Load a file into the current REPL session..load ./file/to/load.js
The following key combinations in the REPL have these special effects:
<ctrl>C
- Similar to the.break
keyword. Terminates the current command. Press twice on a blank line to forcibly exit.<ctrl>D
- Similar to the.exit
keyword.