# How to use the Node.js REPL

> Learn how to use the Node.js REPL, the Read-Evaluate-Print-Loop, to run JavaScript interactively with tab autocomplete, the _ variable, and dot commands.

Author: [Flavio Copes](https://flaviocopes.com/about/) | Published: 2018-08-14 | Topics: [Node.js](https://flaviocopes.com/tags/node/) | Canonical: https://flaviocopes.com/node-repl/

The `node` command is the one we use to run our [Node.js](https://flaviocopes.com/nodejs/) scripts:

```bash
node script.js
```

If we omit the filename, we use it in REPL mode:

```bash
node
```

If you try it now in your terminal, this is what happens:

```
❯ node
>
```

the command stays in idle mode and waits for us to enter something.

> Tip: if you are unsure how to open your terminal, google "How to open terminal on <your-operating-system>".

The REPL is waiting for us to enter some [JavaScript](https://flaviocopes.com/javascript/) code, to be more precise.

Start simple and enter

```
> console.log('test')
test
undefined
>
```

The first value, `test`, is the output we told the console to print, then we get undefined which is the return value of running `console.log()`.

We can now enter a new line of JavaScript.

## Use the tab to autocomplete

The cool thing about the REPL is that it's interactive.

As you write your code, if you press the `tab` key the REPL will try to autocomplete what you wrote to match a variable you already defined or a predefined one.

## Exploring JavaScript objects

Try entering the name of a JavaScript class, like `Number`, add a dot and press `tab`.

The REPL will print all the properties and methods you can access on that class:

![Pressing tab reveals object properties](https://flaviocopes.com/images/node-repl/tab.png)

## Explore global objects

You can inspect the globals you have access to by typing `global.` and pressing `tab`:

![Globals](https://flaviocopes.com/images/node-repl/globals.png)

## The _ special variable

If after some code you type `_`, that is going to print the result of the last operation.

## Dot commands

The REPL has some special commands, all starting with a dot `.`. They are

- `.help`: shows the dot commands help
- `.editor`: enables editor more, to write multiline JavaScript code with ease. Once you are in this mode, enter ctrl-D to run the code you wrote.
- `.break`: when inputting a multi-line expression, entering the .break command will abort further input. Same as pressing ctrl-C.
- `.clear`: resets the REPL context to an empty object and clears any multi-line expression currently being input.
- `.load`: loads a JavaScript file, relative to the current working directory
- `.save`: saves all you entered in the REPL session to a file (specify the filename)
- `.exit`: exists the repl (same as pressing ctrl-C two times)

The REPL knows when you are typing a multi-line statement without the need to invoke `.editor`.

For example if you start typing an iteration like this:

```js
[1, 2, 3].forEach(num => {
```

and you press `enter`, the REPL will go to a new line that starts with 3 dots, indicating you can now continue to work on that block.

```js
... console.log(num)
... })
```

If you type `.break` at the end of a line, the multiline mode will stop and the statement will not be executed.