Node by example

Node by example: 11. REPL

Published on April 29, 2010
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11. REPL

The complete source code can be downloaded here: http://github.com/Hendrik/node-by-example

A Read-Eval-Print-Loop 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.

Start a REPL via:

repl.start(prompt, stream)

where "prompt" is optional and defaults to "node> "
"stream" is options as well and defaults to process.openStdin().
Inside the REPL, Control+D will exit.

 

The following example will start a tcp server listening to port 8000 and we will use the REPL to check & modify the available variables:
See 12_repl/repl.js:

var sys = require("sys"),
    net = require("net"),
   repl = require("repl");

no_connections = 0;
some_var = "Hello";
net.createServer(function(socket) {
  sys.puts("Connection!");
  no_connections += 1;
  socket.end();
}).listen(8000);

repl.start("repl.js prompt> ");

 

First make sure to include the repl module:

var repl = require("repl");

 

Next we set 2 variables, which we will manipulate through the REPL:

no_connections = 0;
some_var = "Hello";

 

Any client connecting to the tcp server will increase the "no_connections" variable:

net.createServer(function(socket) {
  sys.puts("Connection!");
  no_connections += 1;
  socket.end();
}).listen(8000);

 

Start the example via:

node repl.js

 

You will notice that instead of a running process you are presented with a prompt:

# node repl.js
repl.js prompt>

 

Enter the following to get the value of the no_connections variable:

repl.js prompt> no_connections

 

Increase it one or more times and check again:

repl.js prompt> ++no_connections

 

As you can see the REPL is very useful for debugging and testing your node scripts.