Getting Started with REPL
To start using the REPL in Node.js, follow these steps:
Step 1: Install Node.js
Ensure that Node.js is installed on your system. You can check this by running the following command in your terminal:
node -v
If Node.js is installed, this command will return the version number.
Step 2: Open REPL
To access the REPL environment, type node in your terminal and press Enter. You will see the REPL prompt (>), indicating that it’s ready to accept JavaScript code.
$ node
>
Step 3: Execute JavaScript Code
You can now start typing JavaScript expressions or commands. For example:
> console.log('Hello, REPL!');
Hello, REPL!
undefined
Here, the REPL executes console.log() to print the string Hello, REPL!. The undefined output indicates the return value of console.log().
Step 4: Exit REPL
To exit the REPL, type .exit or press Ctrl + C twice.
Key Features of Node.js REPL
1. Immediate Feedback
The REPL evaluates and displays the result of any JavaScript expression you type instantly. This is useful for testing small snippets of code without creating files.
Example:
> 100 * 2
200
2. Multi-line Code Execution
You can enter multi-line code in the REPL by using curly braces {}. This allows you to test more complex logic interactively.
Example:
> {
... let a = 10;
... let b = 20;
... a + b;
... }
30
Here, the REPL evaluates the entire block and returns the result.
3. Variable and Function Definitions
You can define variables and functions in the REPL and use them as needed.
Example:
> let greet = (name) => `Hello, ${name}!`;
undefined
> greet('Alice');
'Hello, Alice!'
4. Tab Completion
REPL offers tab completion, making it easier to explore available methods and properties. For instance, typing con and pressing Tab will suggest console.
5. Saving and Loading Code
The REPL allows you to save your session to a file and reload it later. This is useful for preserving code snippets.
Commands:
> .save filename.js # Saves the current session to filename.js
> .load filename.js # Loads the session from filename.js
Built-in REPL Commands
The Node.js REPL provides several built-in commands to enhance usability. These commands begin with a . and are not part of the JavaScript language itself.
1. .help
Displays a list of all REPL commands.
> .help
Output:
.break Exit from multi-line expressions
.clear Clear the REPL context
.exit Exit the REPL
.help Show REPL commands
.load Load JavaScript from a file
.save Save all REPL commands to a file
2. .break
Exits a multi-line expression. If you’re stuck in an incomplete code block, use .break.
Example:
> {
... let x = 10
... .break
>
3. .clear
Clears the current REPL context, removing all variables and functions.
> .clear
This is useful for starting fresh without exiting the REPL.
4. .exit
Exits the REPL environment.
> .exit
5. .load
Loads JavaScript code from a file into the REPL.
> .load example.js
6. .save
Saves the current REPL session to a file.
> .save session.js
Mastering REPL Shortcuts
REPL in Node.js is not just a tool for running code; it’s also packed with shortcuts & features that can make your coding experience faster & more efficient. These shortcuts help you navigate, edit, & execute code without leaving the REPL environment. Let’s understand this one by one.
1. Starting REPL
To start REPL, open your terminal & type `node`. This will launch the REPL environment, & you’ll see a `>` prompt where you can start typing JavaScript code.
$ node
>
Once you’re inside REPL, you can start typing any valid JavaScript code, & it will execute immediately. For example:
> console.log("Hello, REPL!");
Hello, REPL!
undefined
The `undefined` you see is the return value of the `console.log` statement. REPL always shows the result of the last executed expression.
2. Navigating REPL
REPL provides several keyboard shortcuts to help you navigate & edit your code efficiently. Here are some of the most useful ones:
- Up/Down Arrow Keys: Use these keys to cycle through your command history. This is helpful when you want to reuse or modify a previous command.
- Tab Key: Pressing the Tab key will autocomplete your code. For example, if you type `cons` & press Tab, REPL will automatically complete it to `console`.
- Ctrl + C: This shortcut stops the current command. If you’re stuck in a loop or waiting for input, this will cancel the operation & bring you back to the REPL prompt.
- Ctrl + D: This exits the REPL environment & returns you to the terminal.
3. Using Underscore (`_`) for Last Result
REPL provides a special variable `_` (underscore) that stores the result of the last executed expression. This is helpful when you want to reuse the result of a previous calculation.
> 5 + 5
10
> _ + 10
20
In this example, `_` holds the value `10` from the previous operation, & adding `10` to it gives `20`.
4. Multi-line Code Execution
REPL allows you to write multi-line code, such as functions or loops. To start a multi-line block, simply press Enter after an opening brace `{`. REPL will automatically detect that you’re writing a multi-line statement & will show `...` for each new line.
> function greet(name) {
... console.log(`Hello, ${name}!`);
... }
undefined
> greet("Alice");
Hello, Alice!
undefined
Here, the function `greet` is defined over multiple lines, & then it’s called with the argument `"Alice"`.
Frequently Asked Questions
What is REPL in Node.js?
REPL stands for Read-Eval-Print-Loop. It is an interactive shell in Node.js that allows you to execute JavaScript code directly in the terminal.
How do I exit the REPL in Node.js?
You can exit the REPL by typing .exit or pressing Ctrl + C twice.
Can I save my REPL session?
Yes, you can save your session to a file using the .save command and reload it later using .load.
Conclusion
In this article, we discussed the REPL environment in Node.js, its key features, and built-in commands. The REPL is an excellent tool for developers to quickly test JavaScript code, debug logic, and experiment interactively.
27+ registered 
