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## REPL
A Read-Eval-Print-Loop (REPL) is available both as a standalone program and easily
includable in other programs. 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 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(prompt='> ', stream=process.stdin, eval=eval, useGlobal=false)
Starts a REPL with `prompt` as the prompt and `stream` for all I/O. `prompt`
is optional and defaults to `> `. `stream` is optional and defaults to
`process.stdin`. `eval` is optional too and defaults to async wrapper for `eval`.
If `useGlobal` is set to true, then the repl will use the global object,
instead of running scripts in a separate context.
You can use your own `eval` function if it has following signature:
function eval(cmd, 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("node via stdin> ");
net.createServer(function (socket) {
connections += 1;
repl.start("node via Unix socket> ", socket);
}).listen("/tmp/node-repl-sock");
net.createServer(function (socket) {
connections += 1;
repl.start("node via TCP socket> ", socket);
}).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.
### REPL Features
Inside the REPL, Control+D will exit. Multi-line expressions can be input.
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.
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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