run

JavaScript

Dynamic, high-level programming language for web and server-side development

Overview

JavaScript is the language of the web, powering interactive websites and modern web applications. With run, you can execute JavaScript code using Node.js without setting up npm projects or managing package.json files.

The JavaScript engine in run uses Node.js as the runtime, providing access to all Node.js built-in modules and APIs. The REPL mode is stateful, meaning variables and functions persist across commands within the same session.

Language Aliases

You can invoke JavaScript using any of these aliases:

Supported Aliases
run javascript "console.log('Hello')"
run js "console.log('Hello')"
run node "console.log('Hello')"
run nodejs "console.log('Hello')"
Output
Hello
Hello
Hello
Hello

Basic Usage - Inline Code

Execute JavaScript code directly from the command line:

Simple Console Log
run js "console.log('Hello, World!')"
Output
Hello, World!
Arithmetic and Variables
run js "const x = 10; const y = 20; console.log(x + y)"
Output
30
Template Literals
run js "const name = 'Alice'; console.log(`Hello, ${name}!`)"
Output
Hello, Alice!
Arrow Functions
run js "const square = x => x * x; console.log(square(5))"
Output
25
Array Methods
run js "const nums = [1,2,3,4,5]; console.log(nums.map(x => x * 2))"
Output
[ 2, 4, 6, 8, 10 ]

File Execution

Execute JavaScript files with Node.js:

Execute JS File
# Create a JavaScript file
echo "console.log('Hello from file!')" > hello.js

# Execute with language specified
run js hello.js

# Or let run auto-detect from extension
run hello.js
Output
Hello from file!
Module Example
# Create a module file
cat > math.js << 'EOF'
function factorial(n) {
  if (n <= 1) return 1;
  return n * factorial(n - 1);
}

for (let i = 0; i <= 10; i++) {
  console.log(`${i}! = ${factorial(i)}`);
}
EOF

run js math.js
Output
0! = 1
1! = 1
2! = 2
3! = 6
4! = 24
5! = 120
6! = 720
7! = 5040
8! = 40320
9! = 362880
10! = 3628800

REPL Mode - Interactive JavaScript

Start an interactive JavaScript REPL with 'run js'. The REPL is stateful within the session - variables and functions persist across commands at the js>>> prompt:

Terminal
$ run js
run universal REPL. Type :help for commands.
js>>> let x = 10
undefined
js>>> let y = 20
undefined
js>>> x + y
30
js>>> const greet = name => `Hello, ${name}!`
undefined
js>>> greet('World')
'Hello, World!'
js>>> const numbers = [1, 2, 3, 4, 5]
undefined
js>>> numbers.reduce((a, b) => a + b, 0)
15

REPL Behavior - Stateful

JavaScript's REPL is STATEFUL within a single session:

• Start REPL once with 'run js'

• Variables and functions persist at the js>>> prompt

• Each separate 'run js "code"' command is independent

Node.js Built-in Modules

Access all Node.js built-in modules without installation:

File System Operations
run js "
const fs = require('fs');
const files = fs.readdirSync('.');
console.log(`Found ${files.length} files`);
files.slice(0, 5).forEach(f => console.log(`  - ${f}`));
"
Output
Found 12 files
  - README.md
  - src
  - Cargo.toml
  - target
  - .git
Path Operations
run js "
const path = require('path');
const file = '/home/user/documents/file.txt';
console.log('Directory:', path.dirname(file));
console.log('Filename:', path.basename(file));
console.log('Extension:', path.extname(file));
"
Output
Directory: /home/user/documents
Filename: file.txt
Extension: .txt
OS Information
run js "
const os = require('os');
console.log('Platform:', os.platform());
console.log('CPU Cores:', os.cpus().length);
console.log('Total Memory:', Math.round(os.totalmem() / 1024 / 1024 / 1024) + ' GB');
"
Output
Platform: linux
CPU Cores: 8
Total Memory: 16 GB

Modern JavaScript Features

Use ES6+ features including async/await, destructuring, and more:

Async/Await
run js "
const delay = ms => new Promise(resolve => setTimeout(resolve, ms));
(async () => {
  console.log('Starting...');
  await delay(1000);
  console.log('Done after 1 second!');
})();
"
Output
Starting...
Done after 1 second!
Destructuring
run js "
const person = { name: 'Alice', age: 30, city: 'NYC' };
const { name, age } = person;
console.log(`${name} is ${age} years old`);
"
Output
Alice is 30 years old
Spread Operator
run js "
const arr1 = [1, 2, 3];
const arr2 = [4, 5, 6];
const combined = [...arr1, ...arr2];
console.log(combined);
"
Output
[ 1, 2, 3, 4, 5, 6 ]

Common Use Cases

• Quick JavaScript prototyping and testing

• Node.js script development

• JSON data processing and manipulation

• File system operations and automation

• Testing algorithms and data structures

• Learning modern JavaScript features

• API testing and HTTP requests

• String manipulation and parsing

Limitations

• External npm packages must be installed separately

• No automatic package.json or node_modules management

• Browser-specific APIs (DOM, window) are not available

• Some async operations may require explicit handling