Creating and Running .js Files
This page explains how to save JavaScript code as a text file and run it. The file itself is simply a plain text file saved with a .js extension.
How to write a .js file
Write your JavaScript code in a text editor and save the file with a .js extension. Save the file using the UTF-8 character encoding.
var name = 'Ayanami Rei';
var age = 14;
console.log('Name: ' + name);
console.log('Age: ' + age);
if (age >= 18) {
console.log('Adult.');
} else {
console.log('Minor.');
}
Write JavaScript code directly in the text file and save it like this.
How to write comments
You can write comments (notes) in a .js file. Comments are ignored by the JavaScript engine, so they are useful for leaving descriptions or notes about the code.
| Syntax | Description |
|---|---|
| // comment | A single-line comment. Write it with one space after //. Everything from // to the end of the line is a comment. |
| /* comment */ | A multi-line comment. Everything from /* to */ is a comment. |
// A script that displays user information.
function greet(name, age) {
// Receives name and age as arguments and displays the information.
console.log('Name: ' + name);
console.log('Age: ' + age);
}
greet('Ikari Shinji', 14);
Single-line comments (//) can be used from the middle of a line. Use /* */ for multi-line comments.
How to run (Node.js)
With Node.js, you can run .js files directly from the terminal without using a browser.
node hello.js
Output
Name: Ayanami Rei Age: 14 Minor.
If Node.js is not installed, see Setup.
How to run (browser)
To use a .js file in a browser, load it from inside an HTML file using a <script> tag.
<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="UTF-8">
<title>Sample</title>
</head>
<body>
<h1>Sample Page</h1>
<script src="hello.js"></script>
</body>
</html>
Use <script src="filename.js"></script> to load the .js file. Opening index.html in a browser will execute the JavaScript.
To check the output, use the browser developer tools. Open developer tools with F12 on Windows or Command + Option + I on macOS, then select the Console tab. The output from console.log() will be displayed there.
Summary
A .js file is simply a plain text file. You can create one by writing JavaScript code in a text editor and saving it with a .js extension. No special tools are needed.
There are two ways to run the code. With Node.js you can run it directly from the terminal; in a browser, you load it with the HTML <script> tag. Choose the approach that fits your purpose.
For recommended editors and Node.js installation instructions, see Setup.
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