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# 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 and `stream` for all I/O. Defaults to `> `.
- `input` - the readable stream to listen to. Defaults to `process.stdin`.
- `output` - the writable stream to write readline data to. Defaults to
`process.stdout`.
- `terminal` - pass `true` if the `stream` should be treated like a TTY, and
have ANSI/VT100 escape codes written to it. Defaults to checking `isTTY`
on the `output` stream upon instantiation.
- `eval` - function that will be used to eval each given line. Defaults to
an async wrapper for `eval()`. See below for an example of a custom `eval`.
- `useColors` - a boolean which specifies whether or not the `writer` function
should output colors. If a different `writer` function is set then this does
nothing. Defaults to the repl's `terminal` value.
- `useGlobal` - if set to `true`, then the repl will use the `global` object,
instead of running scripts in a separate context. Defaults to `false`.
- `ignoreUndefined` - if set to `true`, then the repl will not output the
return value of command if it's `undefined`. Defaults to `false`.
- `writer` - the function to invoke for each command that gets evaluated which
returns the formatting (including coloring) to display. Defaults to
`util.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
<!-- type=misc -->
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 the `context` 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.
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