# REPL Terminal

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### REPL (Read Eval Print Loop)

> Refers to an environment where the system returns a value when the user enters a command, similar to the Windows command prompt or UNIX/LINUX Shell

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Node.js comes with a REPL environment and can perform the following functions

* **Read**

  : Reads the user's input, converts it into JavaScript data structures, and stores it in memory
* **Eval**

  : Evaluates (processes) the data
* **Print**

  : Prints the result
* **Loop**

  : Repeats Read, Eval, Print until the user presses `ctrl + c` twice to exit

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*Node.js's REPL environment is useful for testing & debugging JavaScript code!*

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### Starting REPL

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> REPL can be started by running node without any parameters in the Shell / Console

```bash
$ node
>
```

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#### 1. Using simple expressions

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```shell
$ node
> 1 + 5
6
> 1 + ( 6 / 2 ) - 3
1
>
```

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#### 2. Using variables

* Like other scripts, you can store values in variables and print them later
* When using the `var` keyword
* The value of the variable is not printed when the command is entered,
* When the `var` keyword is not used
  * The value of the variable is printed
* You can print using **console.log()**

<br>

```shell
$ node
> x = 10
10
> var y = 5
undefined
> x + y
15
> console.log("Value is " + ( x + y ))
Value is 15
undefined
```

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#### 3. Using Multi-line Expressions

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> Run a do-while loop in REPL!

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

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#### Underscore(\_) Variable

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> The underscore variable refers to the most recent result!

```shell
$ node
> var x = 10;
undefined
> var y = 5;
undefined
> x + y;
15
> var sum = _
undefined
> console.log(sum)
15
undefined
>
```

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### REPL Commands

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* `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`

  : 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

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### Stopping REPL

> As mentioned above, use **ctrl + c** twice to come out of Node.js REPL

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

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