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129 lines
4.1 KiB
Markdown
129 lines
4.1 KiB
Markdown
## REPL
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A Read-Eval-Print-Loop (REPL) is available both as a standalone program and easily
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includable in other programs. REPL provides a way to interactively run
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JavaScript and see the results. It can be used for debugging, testing, or
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just trying things out.
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By executing `node` without any arguments from the command-line you will be
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dropped into the REPL. It has simplistic emacs line-editing.
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mjr:~$ node
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Type '.help' for options.
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> a = [ 1, 2, 3];
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[ 1, 2, 3 ]
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> a.forEach(function (v) {
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... console.log(v);
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... });
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1
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2
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3
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For advanced line-editors, start node with the environmental variable `NODE_NO_READLINE=1`.
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This will start the REPL in canonical terminal settings which will allow you to use with `rlwrap`.
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For example, you could add this to your bashrc file:
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alias node="env NODE_NO_READLINE=1 rlwrap node"
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### repl.start([prompt], [stream], [eval], [useGlobal], [ignoreUndefined])
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Starts a REPL with `prompt` as the prompt and `stream` for all I/O. `prompt`
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is optional and defaults to `> `. `stream` is optional and defaults to
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`process.stdin`. `eval` is optional too and defaults to async wrapper for
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`eval()`.
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If `useGlobal` is set to true, then the repl will use the global object,
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instead of running scripts in a separate context. Defaults to `false`.
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If `ignoreUndefined` is set to true, then the repl will not output return value
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of command if it's `undefined`. Defaults to `false`.
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You can use your own `eval` function if it has following signature:
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function eval(cmd, callback) {
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callback(null, result);
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}
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Multiple REPLs may be started against the same running instance of node. Each
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will share the same global object but will have unique I/O.
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Here is an example that starts a REPL on stdin, a Unix socket, and a TCP socket:
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var net = require("net"),
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repl = require("repl");
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connections = 0;
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repl.start("node via stdin> ");
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net.createServer(function (socket) {
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connections += 1;
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repl.start("node via Unix socket> ", socket);
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}).listen("/tmp/node-repl-sock");
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net.createServer(function (socket) {
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connections += 1;
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repl.start("node via TCP socket> ", socket);
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}).listen(5001);
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Running this program from the command line will start a REPL on stdin. Other
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REPL clients may connect through the Unix socket or TCP socket. `telnet` is useful
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for connecting to TCP sockets, and `socat` can be used to connect to both Unix and
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TCP sockets.
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By starting a REPL from a Unix socket-based server instead of stdin, you can
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connect to a long-running node process without restarting it.
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### REPL Features
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Inside the REPL, Control+D will exit. Multi-line expressions can be input.
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Tab completion is supported for both global and local variables.
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The special variable `_` (underscore) contains the result of the last expression.
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> [ "a", "b", "c" ]
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[ 'a', 'b', 'c' ]
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> _.length
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3
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> _ += 1
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4
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The REPL provides access to any variables in the global scope. You can expose
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a variable to the REPL explicitly by assigning it to the `context` object
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associated with each `REPLServer`. For example:
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// repl_test.js
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var repl = require("repl"),
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msg = "message";
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repl.start().context.m = msg;
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Things in the `context` object appear as local within the REPL:
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mjr:~$ node repl_test.js
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> m
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'message'
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There are a few special REPL commands:
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- `.break` - While inputting a multi-line expression, sometimes you get lost
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or just don't care about completing it. `.break` will start over.
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- `.clear` - Resets the `context` object to an empty object and clears any
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multi-line expression.
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- `.exit` - Close the I/O stream, which will cause the REPL to exit.
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- `.help` - Show this list of special commands.
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- `.save` - Save the current REPL session to a file
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>.save ./file/to/save.js
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- `.load` - Load a file into the current REPL session.
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>.load ./file/to/load.js
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The following key combinations in the REPL have these special effects:
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- `<ctrl>C` - Similar to the `.break` keyword. Terminates the current
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command. Press twice on a blank line to forcibly exit.
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- `<ctrl>D` - Similar to the `.exit` keyword.
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