# Understanding NodeJS REPL

The Node.js REPL (Read-Eval-Print-Loop) is an interactive programming environment that reads input code, parses and executes it, and prints the result, allowing users to try out small pieces of code and address issues in a debugger.

Full form of REPL is given below:

Read - Reads user's input, parses the input into JavaScript data structure, and stores it in memory.

Eval - Takes and evaluates the data structure.

Print - Prints the result.

Loop - Loops the above command until the user presses ctrl-c twice.

### Using REPL in VS Code Terminal

* To use the Node.js REPL, type "node" into a command-line interface like PowerShell or Terminal, and you should see a ">" prompt. Anything typed after this prompt will be parsed and executed as JavaScript.
    
    ```powershell
     node
    ```
    
* The REPL tool is included with Node.js and is a way to interactively run JavaScript and see the results immediately.
    

### Actions that can be performed using REPL

NodeJS REPL can be used to do the following tasks:

1. **Mathematical Expressions:** It can be used to evaluate mathematical expressions.Here's an example of evaluating JavaScript expressions in the Node.js REPL:
    
    ```powershell
    >1+1
    2
    >10*5
    50
    ```
    
2. **Exposing Variables to the REPL:** It is possible to expose a variable to the REPL. Here's an example:
    
    ```powershell
    > const msg = 'message';
    > msg
    'message'
    ```
    
3. **Multiline Code:** Multi-line Javascript code can also be written using the NodeJS REPL.
    
    ```powershell
    > var x=5;
    undefined
    > x
    5
    > for(var i=0; i<x; i++){
    ... console.log("Node REPL");
    ... }
    Node REPL
    Node REPL
    Node REPL
    Node REPL
    Node REPL
    undefined
    > 
    ```
    
4. **Fetch Last Result:** To fetch the last result in the Node.js REPL, you can use the `_` variable. This variable holds the result of the last operation in the REPL. Here is an example:
    
    ```powershell
    > 2 + 2
    4
    > 'hello'
    'hello'
    > _
    'hello'
    ```
    
5. **Entering Editor Mode:** To enter editor mode in the Node.js REPL, you can use the `.editor` command. This command allows you to write JavaScript code that goes beyond one line directly in the REPL. Write your code in the editor and save it by pressing `Ctrl + D` on a new line when you're done.
    
    ```powershell
    $ node
    > .editor
    // Entering editor mode (^D to finish, ^C to cancel)
    function greet(name) {
      console.log(`Hello, ${name}!`);
    }
    greet('Abhinav');
    // Press Ctrl + D to save and exit editor mode
    ```