# Node.js - REPL Terminal

REPL stands for Read Eval Print Loop and it represents a computer environment like a Windows console or Unix/Linux shell where a command is entered and the system responds with an output in an interactive mode. Node.js or **Node** comes bundled with a REPL environment. It performs the following tasks −

- **Read** − Reads user's input, parses the input into JavaScript data-structure, and stores 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.

The REPL feature of Node is very useful in experimenting with Node.js codes and to debug JavaScript codes.

## Online REPL Terminal

To simplify your learning, we have set up an easy to use Node.js REPL environment online, where you can practice Node.js syntax − [Launch Node.js REPL Terminal ](https://www.tutorialspoint.com/nodejs_terminal_online.php "Node.js Terminal Online")

### Starting REPL

REPL can be started by simply running **node** on shell/console without any arguments as follows.

```
$ node
```

You will see the REPL Command prompt > where you can type any Node.js command −

```
$ node
>
```

### Simple Expression

Let's try a simple mathematics at the Node.js REPL command prompt −

```
$ node
> 1 + 3
4
> 1 + ( 2 * 3 ) - 4
3
>
```

### Use Variables

You can make use variables to store values and print later like any conventional script. If **var** keyword is not used, then the value is stored in the variable and printed. Whereas if **var** keyword is used, then the value is stored but not printed. You can print variables using **console.log()**.

```
$ node
> x = 10
10
> var y = 10
undefined
> x + y
20
> console.log("Hello World")
Hello World
undefined
```

### Multiline Expression

Node REPL supports multiline expression similar to JavaScript. Let's check the following do-while loop in action −

```
$ node
> var x = 0
undefined
> do {
   ... x++;
   ... console.log("x: " + x);
   ... } 
while ( x < 5 );
x: 1
x: 2
x: 3
x: 4
x: 5
undefined
>
```

**...** comes automatically when you press Enter after the opening bracket. Node automatically checks the continuity of expressions.

### Underscore Variable

You can use underscore **(\_)** to get the last result −

```
$ node
> var x = 10
undefined
> var y = 20
undefined
> x + y
30
> var sum = _
undefined
> console.log(sum)
30
undefined
>
```

## REPL Commands

- **ctrl + c** − terminate the current command.
- **ctrl + c twice** − terminate the Node REPL.
- **ctrl + d** − terminate the Node REPL.
- **Up/Down Keys** − see command history and modify previous commands.
- **tab Keys** − list of current commands.
- **.help** − list of all commands.
- **.break** − exit from multiline expression.
- **.clear** − exit from multiline expression.
- **.save *filename*** − save the current Node REPL session to a file.
- **.load *filename*** − load file content in current Node REPL session.

## Stopping REPL

As mentioned above, you will need to use **ctrl-c twice** to come out of Node.js REPL.

```
$ node
>
(^C again to quit)
>
```