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Node for Front-End Developers by Garann Means

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REPL

REPL stands for Read-Eval-Print-Loop, and is a utility included with Node that can be very handy when you’re getting started. It works like the debug console in your browser and you can execute whatever commands you want. You don’t need to do anything to install it—you’ll 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 don’t 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.

Let’s say you’re researching modules to manage asynchronous code paths in your project. There are numerous modules to handle asynchronous code and flow control—let’s assume you’ve found a couple you’d like to compare, but neither has a documented API (in reality, the one we’re using below has very nice documentation). To get a better picture of what’s 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 it’s 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 you’re in the midst of testing something.

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