REPL stands for Read-Eval-Print-Loop, and is a utility included with
Node that can be very handy when youâre getting started. It works like the
debug console in your browser and you can execute whatever commands you
want. You donât need to do anything to install itâyouâll have it available
from the command line wherever Node is available. Before even creating an
application you can start poking around and see what Node can do by typing
node
on the command line. If you donât specify an application
to run, you get the REPL instead.
After typing node
at the command prompt, you can test a
few simple lines of JavaScript, and they should evaluate as
normal:
> ["Hello","World"].join(" ") 'Hello World' > 2 + 3 5
You can exit the REPL by pressing Ctrl+C.
Letâs say youâre researching modules to manage asynchronous code
paths in your project. There are numerous modules to handle asynchronous
code and flow controlâletâs assume youâve found a couple youâd like to
compare, but neither has a documented API (in reality, the one weâre using
below has very nice documentation). To get a better picture of whatâs
included, you can install them with npm and then use the REPL to
investigate them. If you installed the async
module, for
instance, you could do this:
$ node > var async = require("async"); undefined > console.dir(async);
That will cause the module to spit out a list of its properties, which you can investigate further from that point, should you so desire. Of course you could just read the source, but itâs useful to know you can open the REPL and see exactly what Node sees. You can also use Tab to auto-complete function and variable names within your code, which may be a faster reference than searching online if youâre in the midst of testing something.
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