An Introduction to Nose.JS REPL

man programming computers on several keyboards and screens

In a Javascript runtime environment, Node Js is a framework or library that executes code on Javascript. Javascript is used by developers for server-side scripting and writing command tools. Node Js allows developers to produce content on dynamic web pages and is sent directly to the web browser of the client. Node js allows web applications to be unified around one specific programming language. Javascript was extremely popular among developers for a while but when Node was released in 2009, it transformed the face of modern web development. It was one of the first backend platforms that was deployed which allowed it to compete with well-established languages. Additionally, the platform is both efficient and lightweight. With event-driven architecture, Node.js promotes the production, detection, consumption, and reaction of events.

What is REPL?

Node REPL is a command prompt and a computer environment that is built-in and accompanies Node at the time of installation. It is often referred to as a console window or programming language environment that easily writes and can test code. It interacts with the user by taking as input one expression and the result is executed and returned to the console. REPL stands for Read, Evaluate, Print, Loop and each letter have its functions.

Read- the inputs by users are read and they are then parsed into the data structure of javascript.

Eval- this parsed data structure of javascript is evaluated to find results.

Print- once the evaluation is over, the result is printed.

Loop- after one execution is complete, the initial input command is looped back

This keeps going until a command is passed to terminate it.

One can launch the Node.js REPL environment online and code can be run on any shell or console.

Commands of REPL-

There are a few commands that REPL uses.

  • .break-
    This command is used when the processing of input needs to be terminated. It is also used to stop the input of more lines in a multi-line expression. Apart from the command, the same function occurs when CTRL+C is pressed.
  • .clear-
    This command is very necessary for REPL since it formats or resets the context so that it goes back to being an empty object. This command also clears the further input of multi-line expressions.
  • .exit-
    As the name suggests, it drives REPL to exit by closing the Input/Output stream. By pressing CTRL+D, the same objective can be fulfilled.
  • .save-
    The ongoing session of REPL is saved in a file.
  • .load-
    When a file is needed during an ongoing REPL session, the .load command is used.
  • .editor-
    If the editor mode is necessary, this command is used. To complete or get out of editor mode CTRL+D is used.
  • .help-
    It is not possible to remember all the commands of  Node.js REPL. So, the .help command shows this list of commands whenever necessary.  This same thing will happen if the TAB key is pressed as well.
  • The REPL command prompt is very useful for practising Node code easily and developers should definitely use it.

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